---
title: "Adaptive Immunity and Vaccination"
book: "High School Biology"
subject: biology
language: en
chapter: 34
exercises: 15
source: https://one-course.com/books/biology/2/en/chapter/34-adaptive-immunity-and-vaccination
---

# Chapter 34 — Adaptive Immunity and Vaccination

In 1796 a country doctor scratched matter from a milkmaid’s cowpox blister into the arm of a boy, waited six weeks, and then inoculated him with smallpox. The boy did not fall ill. Nothing in the doctor’s world explained why; the body, it seemed, had been taught. Two centuries later smallpox is extinct, and the teaching is understood down to the molecules: a set of [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) that carry, between them, a receptor for almost any molecule that could ever exist, that multiply when their molecule appears, and that remember. This chapter describes that system, its two arms, its memory, the [vaccines](#def-g12-adaptive-immunity-vaccine) built on it — and the virus that destroys it.

## 34.1 Antigens and lymphocytes

**Definition 34.1 (Antigen, lymphocyte).**

An *antigen* is any molecule that the adaptive [immune system](https://one-course.com/books/biology/2/en/chapter/33-innate-immunity#def-g12-innate-immunity-immunity) can recognise: typically a [protein](https://one-course.com/books/biology/2/en/chapter/14-from-gene-to-protein#def-g11-gene-expression-protein) or sugar of a microbe, a toxin, a foreign [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell)’s surface — or, in error, one of the body’s own. The recognising [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) are the *lymphocytes*, white [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) made in the bone marrow and gathered in the lymph nodes, the spleen and the lymphoid tissue of the gut. Each lymphocyte carries thousands of copies of one *receptor*, and every receptor binds one antigen only. The *B lymphocytes* mature in the marrow and will make [antibodies](#prop-g12-adaptive-immunity-antibodies); the *T lymphocytes* mature in the thymus and act by contact and by signals.

**Proposition 34.2 (A repertoire made in advance).**

The receptor of each [lymphocyte](#def-g12-adaptive-immunity-antigen) is assembled, while the [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) matures, by a random rearrangement of [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) segments, different in every [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell). The body thus holds some $10^{8}$ different receptors before ever meeting an [antigen](#def-g12-adaptive-immunity-antigen) — enough that nearly any molecule finds a few [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) that bind it. [Lymphocytes](#def-g12-adaptive-immunity-antigen) whose receptor binds the body’s own molecules are eliminated during maturation; the rest wait, a few [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) for each possible [antigen](#def-g12-adaptive-immunity-antigen), until their [antigen](#def-g12-adaptive-immunity-antigen) arrives.

**Proof.** *Admitted at this level.* ∎

![A lymphocyte, in a false-coloured electron micrograph: a small round cell with a rough surface, one of a hundred million kinds, each carrying receptors for a single antigen.](https://one-course.com/images/onecourse/chapters/biology-2/g12-adaptive-immunity/img-e58d7b0f081d.jpg)

*A [lymphocyte](#def-g12-adaptive-immunity-antigen), in a false-coloured electron micrograph: a small round [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) with a rough surface, one of a hundred million kinds, each carrying receptors for a single [antigen](#def-g12-adaptive-immunity-antigen).*

## 34.2 Clonal selection

**Proposition 34.3 (Selection and expansion).**

When an [antigen](#def-g12-adaptive-immunity-antigen) enters, only the [lymphocytes](#def-g12-adaptive-immunity-antigen) whose receptor fits it respond: they are *selected*, and they multiply by [mitosis](https://one-course.com/books/biology/2/en/chapter/12-the-cell-cycle-and-mitosis#def-g11-cell-cycle-mitosis-cycle) into a clone of thousands of identical [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) over several days — *clonal selection*. The clone then differentiates: most [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) become *effector [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell)* that fight the [antigen](#def-g12-adaptive-immunity-antigen) for a week or two and die; a minority become long-lived *memory cells*, which persist for years and, if the [antigen](#def-g12-adaptive-immunity-antigen) returns, respond faster and in far greater numbers. The response is specific because only the fitting clones expand; it is slow the first time because they start from a handful of [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell); it is fast the second time because the [memory cells](#prop-g12-adaptive-immunity-selection) are already many.

**Evidence.** [Antibodies](#prop-g12-adaptive-immunity-antibodies) against a given [antigen](#def-g12-adaptive-immunity-antigen) appear in the blood about a week after a first injection, rise for two weeks and decline; a second injection months later produces, within two or three days, [antibodies](#prop-g12-adaptive-immunity-antibodies) at ten to a hundred times the first peak, which persist for years. Injecting a second, unrelated [antigen](#def-g12-adaptive-immunity-antigen) at the same time as the second dose gives, for that [antigen](#def-g12-adaptive-immunity-antigen), a slow first-type response: the memory is specific. [Lymphocytes](#def-g12-adaptive-immunity-antigen) taken from an immunised animal and transferred to a naive one transfer the memory; those from a non-immunised animal do not. ∎

![Clonal selection. Of the many lymphocyte clones held in reserve, the antigen selects the one whose receptor fits; it multiplies over days into effector cells that fight and memory cells that persist. A second meeting finds the memory clone already large.](https://one-course.com/images/onecourse/chapters/biology-2/g12-adaptive-immunity/fig-9e616be4096d.svg)

*[Clonal selection](#prop-g12-adaptive-immunity-selection). Of the many [lymphocyte](#def-g12-adaptive-immunity-antigen) clones held in reserve, the [antigen](#def-g12-adaptive-immunity-antigen) selects the one whose receptor fits; it multiplies over days into effector [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) that fight and [memory cells](#prop-g12-adaptive-immunity-selection) that persist. A second meeting finds the memory clone already large.*

![Antibodies after two doses of the same antigen. The primary response starts after a week and stays modest; the secondary, from memory cells, starts within days and reaches ten times the level — the principle of the booster dose.](https://one-course.com/images/onecourse/chapters/biology-2/g12-adaptive-immunity/fig-47a4861b9ff8.svg)

*[Antibodies](#prop-g12-adaptive-immunity-antibodies) after two doses of the same [antigen](#def-g12-adaptive-immunity-antigen). The primary response starts after a week and stays modest; the secondary, from [memory cells](#prop-g12-adaptive-immunity-selection), starts within days and reaches ten times the level — the principle of the booster dose.*

## 34.3 Two arms: antibodies and killer cells

**Proposition 34.4 (B lymphocytes and antibodies).**

A selected B clone differentiates into *plasma [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell)*, factories that secrete thousands of *antibodies* per second: soluble copies of the clone’s receptor, Y-shaped [proteins](https://one-course.com/books/biology/2/en/chapter/14-from-gene-to-protein#def-g11-gene-expression-protein) whose two arms each bind the [antigen](#def-g12-adaptive-immunity-antigen). [Antibodies](#prop-g12-adaptive-immunity-antibodies) do not kill; they *mark*. Bound to a virus or a toxin they block it from entering [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) or acting; bound to a bacterium they coat it, and the coat is what the [phagocytes](https://one-course.com/books/biology/2/en/chapter/33-innate-immunity#def-g12-innate-immunity-phagocytosis) of [Chapter 33](https://one-course.com/books/biology/2/en/chapter/33-innate-immunity#ch-g12-innate-immunity) grip best. [Antigens](#def-g12-adaptive-immunity-antigen) cross-linked by many [antibodies](#prop-g12-adaptive-immunity-antibodies) clump into *immune complexes*, which are cleared by [phagocytosis](https://one-course.com/books/biology/2/en/chapter/33-innate-immunity#def-g12-innate-immunity-phagocytosis). This is the arm against intruders in the blood and fluids.

**Proof.** *Admitted at this level.* ∎

![An antibody and what it does. The two arms bind the antigen; the stem is recognised by phagocytes. Many antibodies binding many antigens form a complex that a phagocyte engulfs whole.](https://one-course.com/images/onecourse/chapters/biology-2/g12-adaptive-immunity/fig-00578b7297c1.svg)

*An [antibody](#prop-g12-adaptive-immunity-antibodies) and what it does. The two arms bind the [antigen](#def-g12-adaptive-immunity-antigen); the stem is recognised by [phagocytes](https://one-course.com/books/biology/2/en/chapter/33-innate-immunity#def-g12-innate-immunity-phagocytosis). Many [antibodies](#prop-g12-adaptive-immunity-antibodies) binding many [antigens](#def-g12-adaptive-immunity-antigen) form a complex that a [phagocyte](https://one-course.com/books/biology/2/en/chapter/33-innate-immunity#def-g12-innate-immunity-phagocytosis) engulfs whole.*

**Proposition 34.5 (T lymphocytes).**

T [lymphocytes](#def-g12-adaptive-immunity-antigen) recognise [antigen](#def-g12-adaptive-immunity-antigen) only as fragments displayed on the surface of another [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) ([Chapter 33](https://one-course.com/books/biology/2/en/chapter/33-innate-immunity#ch-g12-innate-immunity)). Two kinds:

- *Helper T cells* are activated by the [dendritic cells](https://one-course.com/books/biology/2/en/chapter/33-innate-immunity#prop-g12-innate-immunity-handover) that bring the [antigen](#def-g12-adaptive-immunity-antigen) to the lymph node; once selected and multiplied, they secrete cytokines that authorise and amplify the response of the B [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) and of the other T [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) . They are the coordinators; without them, almost no adaptive response occurs.
- *Cytotoxic T [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell)* recognise fragments of a virus (or of a [tumour](https://one-course.com/books/biology/2/en/chapter/17-genome-damage-and-cancer#def-g11-cancer-cancer) [protein](https://one-course.com/books/biology/2/en/chapter/14-from-gene-to-protein#def-g11-gene-expression-protein) ) displayed by an infected [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) , and kill that [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) by contact, before the virus inside it can multiply. This is the arm against intruders hidden inside [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) , where [antibodies](#prop-g12-adaptive-immunity-antibodies) cannot reach.

**Proof.** *Admitted at this level.* ∎

![The organisation of the adaptive response. The dendritic cell’s report activates the helper T cells, whose cytokines authorise the B cells to make antibodies and the cytotoxic T cells to kill; each arm leaves memory cells behind.](https://one-course.com/images/onecourse/chapters/biology-2/g12-adaptive-immunity/fig-45c0f69a76b9.svg)

*The organisation of the adaptive response. The [dendritic cell](https://one-course.com/books/biology/2/en/chapter/33-innate-immunity#prop-g12-innate-immunity-handover)’s report activates the [helper T cells](#prop-g12-adaptive-immunity-tcells), whose cytokines authorise the B [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) to make [antibodies](#prop-g12-adaptive-immunity-antibodies) and the cytotoxic T [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) to kill; each arm leaves [memory cells](#prop-g12-adaptive-immunity-selection) behind.*

**Example 34.6 (A virus, met twice).**

First influenza: the virus multiplies for three days before the adaptive response is ready; by day 7 cytotoxic T [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) are killing infected [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) in the airways and [antibodies](#prop-g12-adaptive-immunity-antibodies) are neutralising free virus; by day 10 the person is recovering, with [memory cells](#prop-g12-adaptive-immunity-selection) of both kinds. The same strain a year later: [antibodies](#prop-g12-adaptive-immunity-antibodies) already in the blood block most of the virus at entry, [memory cells](#prop-g12-adaptive-immunity-selection) expand within two days, and the infection is over before it is noticed. A different strain whose surface [proteins](https://one-course.com/books/biology/2/en/chapter/14-from-gene-to-protein#def-g11-gene-expression-protein) have mutated escapes the [antibodies](#prop-g12-adaptive-immunity-antibodies) — which is why the [vaccine](#def-g12-adaptive-immunity-vaccine) is remade every year.

## 34.4 Vaccination

**Definition 34.7 (Vaccine).**

A *vaccine* is a preparation that provokes a primary adaptive response, and hence memory, against a pathogen without causing its disease: the pathogen weakened (measles, tuberculosis), killed (polio by injection), reduced to one of its [proteins](https://one-course.com/books/biology/2/en/chapter/14-from-gene-to-protein#def-g11-gene-expression-protein) or an inactivated toxin (tetanus, hepatitis B), or, most recently, to the instructions for the [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) to make that [protein](https://one-course.com/books/biology/2/en/chapter/14-from-gene-to-protein#def-g11-gene-expression-protein). An *adjuvant* added to the preparation provokes the small inflammation without which the [dendritic cells](https://one-course.com/books/biology/2/en/chapter/33-innate-immunity#prop-g12-innate-immunity-handover) would not carry the [antigen](#def-g12-adaptive-immunity-antigen) to the lymph node. Booster doses turn the primary response into a secondary one and extend the memory for decades.

![A vaccination: an antigen delivered into the arm, with an adjuvant to raise a local inflammation. Within weeks the lymph nodes hold memory cells against a disease the person has never had.](https://one-course.com/images/onecourse/chapters/biology-2/g12-adaptive-immunity/img-ef94d2c438b6.jpg)

*A vaccination: an [antigen](#def-g12-adaptive-immunity-antigen) delivered into the arm, with an adjuvant to raise a local inflammation. Within weeks the lymph nodes hold [memory cells](#prop-g12-adaptive-immunity-selection) against a disease the person has never had.*

![Edward Jenner, who in 1796 showed that inoculation with cowpox protected against smallpox — vaccination, before any of its mechanism was known. Oil painting, Wellcome Collection, CC BY 4.0.](https://one-course.com/images/onecourse/chapters/biology-2/g12-adaptive-immunity/img-c63fdd3a5404.jpg)

*Edward Jenner, who in 1796 showed that inoculation with cowpox protected against smallpox — vaccination, before any of its mechanism was known. Oil painting, Wellcome Collection, CC BY 4.0.*

**Proposition 34.8 (Protecting the group).**

A vaccinated person is protected; a vaccinated [population](https://one-course.com/books/biology/2/en/chapter/25-selection-drift-and-speciation#def-g12-selection-drift-speciation-population) protects its unvaccinated members too. An infected person passes a disease, on average, to $R_0$ others in a [population](https://one-course.com/books/biology/2/en/chapter/25-selection-drift-and-speciation#def-g12-selection-drift-speciation-population) with no immunity (about 15 for measles, 2 to 3 for influenza); if a fraction $v$ is immune, only $R_0(1 - v)$ of those contacts can be infected, and the disease recedes when that number falls below 1 — that is, when

$$
v > 1 - \frac{1}{R_0}.
$$

For measles, 93% of the [population](https://one-course.com/books/biology/2/en/chapter/25-selection-drift-and-speciation#def-g12-selection-drift-speciation-population) must be immune to protect the infants too young for the [vaccine](#def-g12-adaptive-immunity-vaccine) and the people who cannot receive it; for influenza, about 60%. This *herd immunity* is why vaccination is a collective act.

**Proof.** *Admitted at this level.* ∎

![The fraction of a population that must be immune to stop a disease spreading, against the disease’s R_0. The more contagious the disease, the closer to everyone the coverage must be.](https://one-course.com/images/onecourse/chapters/biology-2/g12-adaptive-immunity/fig-4e95731985f0.svg)

*The fraction of a [population](https://one-course.com/books/biology/2/en/chapter/25-selection-drift-and-speciation#def-g12-selection-drift-speciation-population) that must be immune to stop a disease spreading, against the disease’s $R_0$. The more contagious the disease, the closer to everyone the coverage must be.*

## 34.5 When the coordinator is destroyed

**Proposition 34.9 (HIV and AIDS).**

The human immunodeficiency virus, HIV, infects the [helper T cells](#prop-g12-adaptive-immunity-tcells), entering through the very receptor that marks them, and multiplies in them. For years the [immune system](https://one-course.com/books/biology/2/en/chapter/33-innate-immunity#def-g12-innate-immunity-immunity) kills infected [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) and makes [antibodies](#prop-g12-adaptive-immunity-antibodies) — the person is *seropositive*, and infectious — while the virus, mutating constantly, escapes each response and slowly depletes the helper [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell). When their count falls below about a fifth of normal, the adaptive response can no longer be coordinated: *AIDS*, in which infections that a healthy body clears without noticing — a fungus of the mouth, a mild parasite of the lungs — become fatal, and rare [cancers](https://one-course.com/books/biology/2/en/chapter/17-genome-damage-and-cancer#def-g11-cancer-cancer) appear. Antiviral drugs block the virus’s [enzymes](https://one-course.com/books/biology/2/en/chapter/15-enzymes-and-the-phenotype#def-g11-enzymes-and-phenotype-enzyme), restore the helper [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) and, taken for life, hold the disease off and stop transmission; there is no [vaccine](#def-g12-adaptive-immunity-vaccine) yet, and no cure.

**Proof.** *Admitted at this level.* ∎

![The course of an untreated HIV infection (rounded average). After a first burst the virus is held down for years by the immune response while the helper T cells decline; below about 200 per microlitre, the response collapses, the virus surges, and opportunistic infections begin.](https://one-course.com/images/onecourse/chapters/biology-2/g12-adaptive-immunity/fig-468c9eeb1a9b.svg)

*The course of an untreated HIV infection (rounded average). After a first burst the virus is held down for years by the immune response while the [helper T cells](#prop-g12-adaptive-immunity-tcells) decline; below about 200 per microlitre, the response collapses, the virus surges, and opportunistic infections begin.*

![HIV particles (green) budding from the surface of an infected lymphocyte, in a false-coloured electron micrograph. Each particle carries copies of the virus’s genome, and among them the mutants that the next response will not recognise. Micrograph by C. Goldsmith, CDC, public domain.](https://one-course.com/images/onecourse/chapters/biology-2/g12-adaptive-immunity/img-539c9b5a4601.jpg)

*HIV particles (green) budding from the surface of an infected [lymphocyte](#def-g12-adaptive-immunity-antigen), in a false-coloured electron micrograph. Each particle carries copies of the virus’s genome, and among them the mutants that the next response will not recognise. Micrograph by C. Goldsmith, CDC, public domain.*

**Method 34.10 (Reading an immune response).**

1. Identify the [antigen](#def-g12-adaptive-immunity-antigen) and where it is: free in the fluids ( [antibodies](#prop-g12-adaptive-immunity-antibodies) act) or inside [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) (cytotoxic T [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) act).
2. Read the timing: a week to a peak means a primary response, a few days means memory.
3. Read the specificity: a response to one [antigen](#def-g12-adaptive-immunity-antigen) leaves the response to another untouched.
4. Locate the [helper T cells](#prop-g12-adaptive-immunity-tcells) : without them nothing happens, which is what [AIDS](#prop-g12-adaptive-immunity-hiv) demonstrates.
5. For a [vaccine](#def-g12-adaptive-immunity-vaccine) , name the form of the [antigen](#def-g12-adaptive-immunity-antigen) , the adjuvant, the number of doses, and the fraction of the [population](https://one-course.com/books/biology/2/en/chapter/25-selection-drift-and-speciation#def-g12-selection-drift-speciation-population) needed.

**Remark 34.11 (Selection, once more).**

The adaptive [immune system](https://one-course.com/books/biology/2/en/chapter/33-innate-immunity#def-g12-innate-immunity-immunity) is Darwinian: a [population](https://one-course.com/books/biology/2/en/chapter/25-selection-drift-and-speciation#def-g12-selection-drift-speciation-population) of [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) with random receptors, of which the environment — the [antigen](#def-g12-adaptive-immunity-antigen) — selects a few to multiply, and whose descendants inherit the winning receptor. It runs the process of [Chapter 25](https://one-course.com/books/biology/2/en/chapter/25-selection-drift-and-speciation#ch-g12-selection-drift-speciation) inside one body in a week, and, through memory, keeps its results for a lifetime. It is also the reason the [immune system](https://one-course.com/books/biology/2/en/chapter/33-innate-immunity#def-g12-innate-immunity-immunity) can be taught, which is what a [vaccine](#def-g12-adaptive-immunity-vaccine) does, and why a virus that mutates faster than the system can select — HIV, influenza — is so hard to beat.

## 34.6 Exercises

**Exercise 34.1 ★.**

Define [antigen](#def-g12-adaptive-immunity-antigen) and [lymphocyte](#def-g12-adaptive-immunity-antigen), and say what makes each [lymphocyte](#def-g12-adaptive-immunity-antigen) specific.

**Solution of Exercise 34.1.**

An [antigen](#def-g12-adaptive-immunity-antigen) is any molecule the adaptive system can recognise; a [lymphocyte](#def-g12-adaptive-immunity-antigen) is a white [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) carrying receptors for one [antigen](#def-g12-adaptive-immunity-antigen). Its specificity comes from its receptor, assembled at random during maturation and identical on all its copies.

**Exercise 34.2 ★.**

Describe [clonal selection](#prop-g12-adaptive-immunity-selection) in four steps.

**Solution of Exercise 34.2.**

The [antigen](#def-g12-adaptive-immunity-antigen) binds the few [lymphocytes](#def-g12-adaptive-immunity-antigen) whose receptor fits; those [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) multiply into a clone over days; the clone differentiates into effector [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) that fight; a minority become [memory cells](#prop-g12-adaptive-immunity-selection) that persist.

**Exercise 34.3 ★.**

What is an [antibody](#prop-g12-adaptive-immunity-antibodies), what does it bind, and what does it do to a bacterium?

**Solution of Exercise 34.3.**

A Y-shaped [protein](https://one-course.com/books/biology/2/en/chapter/14-from-gene-to-protein#def-g11-gene-expression-protein) secreted by plasma [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell), a soluble copy of the B [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell)’s receptor; each arm binds the [antigen](#def-g12-adaptive-immunity-antigen). On a bacterium it forms a coat that [phagocytes](https://one-course.com/books/biology/2/en/chapter/33-innate-immunity#def-g12-innate-immunity-phagocytosis) grip, and cross-links bacteria into complexes that are engulfed.

**Exercise 34.4 ★.**

Give the roles of helper and cytotoxic T [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell).

**Solution of Exercise 34.4.**

[Helper T cells](#prop-g12-adaptive-immunity-tcells), activated by [dendritic cells](https://one-course.com/books/biology/2/en/chapter/33-innate-immunity#prop-g12-innate-immunity-handover), secrete the cytokines that authorise and amplify the B [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) and the other T [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell). Cytotoxic T [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) kill [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) displaying fragments of a virus or a [tumour](https://one-course.com/books/biology/2/en/chapter/17-genome-damage-and-cancer#def-g11-cancer-cancer) [protein](https://one-course.com/books/biology/2/en/chapter/14-from-gene-to-protein#def-g11-gene-expression-protein).

**Exercise 34.5 ★.**

What is in a [vaccine](#def-g12-adaptive-immunity-vaccine), and why does it work?

**Solution of Exercise 34.5.**

A harmless form of the pathogen’s [antigen](#def-g12-adaptive-immunity-antigen) (weakened, killed, a [protein](https://one-course.com/books/biology/2/en/chapter/14-from-gene-to-protein#def-g11-gene-expression-protein), a toxin inactivated, or its instructions) with an adjuvant. It provokes a primary response and leaves [memory cells](#prop-g12-adaptive-immunity-selection), so that the real pathogen meets a secondary response.

**Exercise 34.6 ★★.**

From the [antibody](#prop-g12-adaptive-immunity-antibodies) figure, compare the delay, the peak and the duration of the primary and secondary responses.

**Solution of Exercise 34.6.**

Primary: a week’s delay, a low peak (about a tenth of the secondary’s) at two to three weeks, a decline within a month. Secondary: two or three days’ delay, a peak ten times higher within ten days, persisting for months.

**Exercise 34.7 ★★.**

A second injection of [antigen](#def-g12-adaptive-immunity-antigen) A together with a first injection of [antigen](#def-g12-adaptive-immunity-antigen) B gives a strong fast response to A and a slow weak one to B. What does this show?

**Solution of Exercise 34.7.**

Memory is specific: the [memory cells](#prop-g12-adaptive-immunity-selection) left by the first dose of A respond to A only; B, met for the first time, gets a primary response.

**Exercise 34.8 ★★.**

Why can [antibodies](#prop-g12-adaptive-immunity-antibodies) not clear a virus once it is inside a [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell), and which [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) can?

**Solution of Exercise 34.8.**

[Antibodies](#prop-g12-adaptive-immunity-antibodies) are [proteins](https://one-course.com/books/biology/2/en/chapter/14-from-gene-to-protein#def-g11-gene-expression-protein) in the fluids and cannot enter [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell). An infected [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) displays fragments of the virus on its surface, and cytotoxic T [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) recognise them and kill the [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell).

**Exercise 34.9 ★★.**

Compute the vaccination coverage needed for a disease with $R_0 = 4$, and with $R_0 = 18$.

**Solution of Exercise 34.9.**

$1 - 1/4 = 75\%$; $1 - 1/18 \approx 94\%$.

**Exercise 34.10 ★★.**

Why does a measles vaccination campaign protect infants who are too young to be vaccinated?

**Solution of Exercise 34.10.**

Above the threshold each case infects fewer than one other, so the virus cannot circulate and rarely reaches an infant; the immune adults are the wall around them.

**Exercise 34.11 ★★.**

From the HIV figure, read the [helper T cell](#prop-g12-adaptive-immunity-tcells) count at 1, 5 and 9 years, and say when [AIDS](#prop-g12-adaptive-immunity-hiv) begins.

**Solution of Exercise 34.11.**

About 750, 520 and 180 per microlitre; [AIDS](#prop-g12-adaptive-immunity-hiv) begins around year 9, when the count crosses 200.

**Exercise 34.12 ★★★.**

Explain why the destruction of [helper T cells](#prop-g12-adaptive-immunity-tcells) alone disables both the [antibody](#prop-g12-adaptive-immunity-antibodies) arm and the [killer-cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) arm, using the organisation figure.

**Solution of Exercise 34.12.**

The [helper T cells](#prop-g12-adaptive-immunity-tcells)’ cytokines are what authorise the B [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) to become plasma [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) and the cytotoxic T [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) to multiply; both arms depend on the same coordinator, so removing it silences both.

**Exercise 34.13 ★★★.**

A seropositive person carries [antibodies](#prop-g12-adaptive-immunity-antibodies) against HIV yet is not protected. Explain why, with the virus’s [mutation](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#def-g11-mutations-mutation) rate and its target.

**Solution of Exercise 34.13.**

The [antibodies](#prop-g12-adaptive-immunity-antibodies) were selected against the virus as it was; the virus mutates its surface [proteins](https://one-course.com/books/biology/2/en/chapter/14-from-gene-to-protein#def-g11-gene-expression-protein) faster than new clones can be selected, so each response is outrun. And the virus hides and multiplies inside the very helper [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) that would coordinate the response.

**Exercise 34.14 ★★★.**

The influenza [vaccine](#def-g12-adaptive-immunity-vaccine) is remade every year; the measles [vaccine](#def-g12-adaptive-immunity-vaccine) of 1970 still works. Explain the difference with the [mutation](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#def-g11-mutations-mutation) of the viruses’ surface [proteins](https://one-course.com/books/biology/2/en/chapter/14-from-gene-to-protein#def-g11-gene-expression-protein) and the specificity of memory.

**Solution of Exercise 34.14.**

[Memory cells](#prop-g12-adaptive-immunity-selection) recognise the surface [proteins](https://one-course.com/books/biology/2/en/chapter/14-from-gene-to-protein#def-g11-gene-expression-protein) the [vaccine](#def-g12-adaptive-immunity-vaccine) presented. Influenza’s mutate every year into forms the old memory does not bind; measles’ surface [proteins](https://one-course.com/books/biology/2/en/chapter/14-from-gene-to-protein#def-g11-gene-expression-protein) hardly change, so the memory of 1970 still fits the virus of today.

**Exercise 34.15 ★★★.**

Compare the adaptive immune response to [natural selection](https://one-course.com/books/biology/2/en/chapter/25-selection-drift-and-speciation#def-g12-selection-drift-speciation-selection) in a [population](https://one-course.com/books/biology/2/en/chapter/25-selection-drift-and-speciation#def-g12-selection-drift-speciation-population): what varies, what selects, what is inherited, and what the time scale is. Where does the analogy stop?

**Solution of Exercise 34.15.**

What varies: the receptors of the [lymphocytes](#def-g12-adaptive-immunity-antigen), made at random. What selects: the [antigen](#def-g12-adaptive-immunity-antigen), which lets only fitting clones multiply. What is inherited: the receptor, by the clone’s descendants, including the [memory cells](#prop-g12-adaptive-immunity-selection). Time scale: days for a response, a lifetime for memory, against generations for a [population](https://one-course.com/books/biology/2/en/chapter/25-selection-drift-and-speciation#def-g12-selection-drift-speciation-population). The analogy stops at transmission: the selected clones die with the body and are not passed to the next generation.

## 34.7 Problem: Ninety-Three Per Cent

**Problem 34.1.**

Weekend problem — a vaccination campaign reckoned: the responses to two doses, the arithmetic of herd immunity, a measles outbreak modelled, and the virus that undoes the system

Measles has $R_0 = 15$. A town of 50 000 people has 90% of its [population](https://one-course.com/books/biology/2/en/chapter/25-selection-drift-and-speciation#def-g12-selection-drift-speciation-population) immune by vaccination or past infection; 1500 are infants under one year, too young for the [vaccine](#def-g12-adaptive-immunity-vaccine).

**Part I — One person’s response.** A child receives the first dose at 12 months and the second at 18.

1. Describe the [antibody](#prop-g12-adaptive-immunity-antibodies) curve after the first dose: delay, peak, decline.
2. Describe it after the second dose and explain the difference with [clonal selection](#prop-g12-adaptive-immunity-selection) .
3. The [vaccine](#def-g12-adaptive-immunity-vaccine) virus is weakened but alive. Why is a living virus used, and which arm of the response does it train that a killed one would not?
4. Ten years later the child meets measles. What happens in the first three days, and why does she not fall ill?
5. Her unvaccinated cousin meets the same virus. Describe his first ten days, and what he keeps afterwards.

**Part II — The threshold.**

6. Compute the fraction of the [population](https://one-course.com/books/biology/2/en/chapter/25-selection-drift-and-speciation#def-g12-selection-drift-speciation-population) that must be immune for measles to stop spreading.
7. With 90% immune, how many people can one infected person infect on average? Is the town protected?
8. How many people in the town are not immune, and how many of them are the infants?
9. If coverage rose to 95%, how many susceptible people would remain, and what would one case produce on average?
10. Explain in one sentence why the infants’ safety depends on the adults’ decisions.

**Part III — An outbreak.** A traveller brings measles into the town at 90% immunity.

11. Take each case to infect $15 \times 0.10 = 1.5$ others, in successive generations of 12 days. How many cases in the third generation? In the sixth?
12. How many cases in all after six generations?
13. Of the 5000 susceptible people, 30% are infants. Estimate the number of infants among the cases after six generations, if cases fall evenly on the susceptible.
14. Each generation of cases leaves immune survivors: explain why the outbreak slows on its own, and roughly when.
15. Repeat question 11 for a town at 95% immunity. Compare the two towns.

**Part IV — The system undone.** A patient infected with HIV, untreated, loses [helper T cells](#prop-g12-adaptive-immunity-tcells) from 1000 to 200 per microlitre in 8 years.

16. Compute the average loss per year, and per month.
17. At 200 per microlitre, [AIDS](#prop-g12-adaptive-immunity-hiv) begins. Explain, with the organisation figure, why both [antibodies](#prop-g12-adaptive-immunity-antibodies) and killer [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) are affected although only helper [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) are infected.
18. The patient’s measles [memory cells](#prop-g12-adaptive-immunity-selection) are intact. Would a measles exposure now be dangerous? Explain.
19. Antiviral treatment started at year 8 raises the count back to 600 within two years. What does this show about the [memory cells](#prop-g12-adaptive-immunity-selection) and the repertoire, and why must the treatment continue?
20. State the result: the coverage that protects the town’s infants, the number of cases one traveller seeds in six generations at 90%, and the one [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) whose loss brings the whole adaptive system down.

**Solution of Problem 34.1.**

**1.** Nothing for about a week, a modest peak at two to three weeks, a decline over the following weeks.

**2.** [Antibodies](#prop-g12-adaptive-immunity-antibodies) within two or three days, a peak ten times higher, lasting months: the [memory cells](#prop-g12-adaptive-immunity-selection) left by the first dose are already a large clone, so multiplication starts from thousands of [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) instead of a few.

**3.** A living virus multiplies briefly inside [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell), so infected [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) display its fragments and cytotoxic T [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) are selected as well as [antibodies](#prop-g12-adaptive-immunity-antibodies); a killed virus trains mainly the [antibody](#prop-g12-adaptive-immunity-antibodies) arm.

**4.** [Antibodies](#prop-g12-adaptive-immunity-antibodies) already present block much of the virus; memory B and T [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) multiply within two days; infected [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) are killed before the virus spreads. The infection is stopped before symptoms.

**5.** Three days of silent multiplication, then fever, rash and illness while the primary response builds over a week; recovery around day ten, with [memory cells](#prop-g12-adaptive-immunity-selection) and lifelong immunity — if he survives the complications.

**6.** $1 - 1/15 \approx 93\%$.

**7.** $15 \times 0.10 = 1.5$: each case infects more than one other, so the disease can spread. Not protected.

**8.** $10\% \times 50\,000 = 5000$ not immune, of whom 1500 are the infants.

**9.** 2500 susceptible; $15 \times 0.05 = 0.75$: fewer than one, the disease dies out.

**10.** Infants cannot be vaccinated, so their only protection is that the adults around them, by being immune, leave the virus no path to reach them.

**11.** Generation 3: $1.5^3 \approx 3.4$ cases; generation 6: $1.5^6 \approx 11$.

**12.** $1 + 1.5 + 2.25 + 3.4 + 5.1 + 7.6 + 11.4 \approx 32$ cases.

**13.** About 30% of 32: some 10 infants.

**14.** Each case removes a susceptible person and adds an immune one, so the susceptible fraction falls and with it the number each case infects; with 5000 susceptible the outbreak would slow only after hundreds of cases, over months — unless vaccination intervenes.

**15.** At 95%: $0.75^3 \approx 0.4$ and $0.75^6 \approx 0.18$ cases; total about $1 + 0.75 + 0.56 + \dots \approx 4$ cases. Ten times fewer cases, and the outbreak extinguishes itself.

**16.** 800 in 8 years: 100 per microlitre per year, about 8 per month.

**17.** B [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) need the helper [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell)’ cytokines to become plasma [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell), and cytotoxic T [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) need them to multiply: with too few helpers, neither arm is authorised, though their [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) exist.

**18.** Yes: [memory cells](#prop-g12-adaptive-immunity-selection) still need helper cytokines to expand into a secondary response; without coordination the memory cannot be used, and measles, harmless to a healthy adult, can be fatal.

**19.** The helper [population](https://one-course.com/books/biology/2/en/chapter/25-selection-drift-and-speciation#def-g12-selection-drift-speciation-population) regrows from surviving [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) and the marrow, and the [memory cells](#prop-g12-adaptive-immunity-selection) of the other arms were never destroyed — the repertoire is intact once coordination returns. The virus persists in hidden [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell), so stopping the drugs lets it resume.

**20.** 93% coverage; about 32 cases from one traveller at 90%; the [helper T cell](#prop-g12-adaptive-immunity-tcells).