Body reacts to antigens when exposed, they retain memory of those antigens and there will be a faster response if exposed a second time. Involves immune cells, t and b cells and antigen processing cells. Recognizes foreign substances a job for major histocompatibility complex. Innate immunity: natural / nonspecific host defences. Nonadaptive (innate immunity: barrier to infection, anatomical, microbiota, nonspecific responses to destroy invading cells, present at birth. Acquired immunity: skin and mouth 1:10 (aerobe : anaerobe ratio), acquired from birth canal and caregiver, gi tract 1:100 (aerobe : anaerobe ratio), acquired from surrounding environment. Intestine 1:1000 (aerobe : anaerobe ratio), acquired from breast milk. Influx of recruitment of pmn"s: redness, swelling, heat and pain, cytokine release complement reactions, stops spreading of infection. Involves pyrogens which are and substance that induces fever: bacterial pyrogens: lps / endotoxin, endogenous pyrogen: il-1, tnfk, il-6, interferon, a protective mechanism against microbes.