What is Lymphocytes and their receptors?

Example 16.7 (Kinetics of a response)

About one B cell in $10^{5}$ binds a given epitope, so a body’s $10^{11}$ B cells hold some $10^{6}$ precursors, of which perhaps a hundred meet the antigen in the draining node. Dividing every eight hours, a hundred cells become $10^{2}\times 2^{21} \approx 2\times 10^{8}$ in a week. Antibody appears in the serum after four to six days, IgM first, peaks at about two weeks and declines with a half-life of three weeks for IgG as the short-lived plasma cells die, settling at the level the long-lived marrow plasma cells maintain for years. A second exposure starts from $10^{4}$–$10^{5}$ memory cells of high affinity already switched to IgG: antibody rises within two days, ten- to a hundredfold higher, and neutralises the pathogen before it can establish itself — which is what a vaccine buys.

Example 16.9 (Hypersensitivity, deficiency, transplantation)

Allergy is a Th2 response to a harmless antigen — pollen, peanut, penicillin — that produces IgE; on re-exposure the antigen cross-links IgE on mast cells, which release histamine within minutes, and if the antigen is in the blood the systemic release is anaphylaxis, treated with adrenaline. Immunodeficiency: children lacking RAG or the enzyme ADA make no lymphocytes (severe combined immunodeficiency) and die of infection unless given marrow or gene therapy; HIV destroys CD4 T cells and with them the help every response needs. Transplantation: the recipient’s T cells see the donor’s HLA molecules as foreign, and up to a tenth of all T cells respond — far more than to any pathogen — so grafts are matched at the HLA loci and the recipient is immunosuppressed for life; a graft of marrow can attack its new host instead. And the deliberate uses: the monoclonal antibodies of Chapter 6, the checkpoint inhibitors that release T cells against tumours (Chapter 11), and T cells engineered with a chimaeric receptor for a tumour antigen (CAR-T), which have cured leukaemias that nothing else touched.

Example 16.12 (Measles and smallpox)

Measles has $R_{0} \approx 15$: $p_{c} = 1 - 1/15 = 93\,\%$, and with a vaccine of efficacy $97\,\%$ after two doses the coverage needed is $96\,\%$ — which is why measles returns wherever vaccination slips a few per cent, and why an unvaccinated child in a well-vaccinated country is nonetheless safe. Smallpox had $R_{0} \approx 5$–$7$, a threshold near $80$–$85\,\%$, no animal reservoir, no asymptomatic carriers and a vaccine that worked in one dose: eradicable, and eradicated. Polio is nearly there. Influenza and the coronaviruses, whose antigens drift (Chapter 13) and whose immunity wanes, are not. In an unvaccinated population an epidemic with $R_{0} = 3$ infects, by the final-size relation, $1 - s_{\infty} = 94\,\%$ of people; with $R_{0} = 1.5$, $58\,\%$.