Disease and immunity
Pathogens cause disease and spread by direct contact (blood, body fluids) or indirectly (air, water, food, vectors). The body's defences: skin, mucus and cilia, stomach acid, then white blood cells. Each pathogen carries antigens; lymphocytes make antibodies with complementary shapes that bind to them, marking pathogens for destruction. Memory cells remain, giving active immunity: a second infection is fought off faster and stronger. Vaccines contain harmless antigens to trigger this. Passive immunity (antibodies from breast milk or injection) is short-lived.
≈ 40 min