What is The standing army?

Patrolling the blood and tissues stand the white blood cells — the immune system’s troops, made in the bone’s marrow, riding Proposition 27.1’s network. The first responders are the engulfers: cells that flow around an intruder and digest it (Example 45.5’s flowing blob ran the same maneuver as a free-living hunter — the body employs the ancient trick as a defense).