The immune system has three great weapons. Phagocytes that eat invaders. Antibodies and complement that tag them. And T cells that reach what hides inside cells. A successful pathogen survives by switching those weapons off one at a time. Equip a microbe with escape tricks, then see which defense still gets through.
The attack on one microbeswitch a trick on and watch
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Each defense, right now
Three defenses, six escape tricksequip the microbe and see who still wins
Escape tricks on this microbe
Host defenses that are switched on
Three escapes, one idea. A capsule hides you from phagocytes. Antigenic variation hides you from antibodies. Going intracellular hides you inside the one place an antibody cannot follow.
Encapsulated organisms, which are pneumococcus, meningococcus, and Hib, can overwhelm a person with no working spleen. That includes sickle cell disease and surgical removal, and it is why those vaccines are given. Hiders such as M. tuberculosis can stay latent for years, then wake up under steroids, TNF blockers, or HIV.