6.12—Evolutionary race between pathogens and hosts

An evolutionary arms race occurs when pathogen variants that evade immunity reproduce more successfully while host defences favour recognition and removal. The changing pathogen and host populations create reciprocal selection pressures.

HIV can vary its antigens, kill helper T cells and reduce antigen presentation; Mycobacterium tuberculosis can interfere with lysosome fusion and persistence inside phagocytes.

Memory cells against one viral strain may not recognise a mutated antigen, so the host faces a new primary-like response while the new strain spreads.

Co-evolution does not mean every mutation is advantageous or that immunity disappears completely. Identify the evasion mechanism and the selection consequence.