ABSTRACT
Immunology is the study of the physiological mechanisms that humans and other animals use to defend their bodies from invasion by all sorts of other organisms. The origins of the subject lie in the practice of medicine and in historical observations that people who survived the ravages of epidemic disease were untouched when faced with that same disease again-they had become immune to infection. Infectious diseases are caused by microorganisms, which have the advantage of reproducing and evolving much more rapidly than their human hosts. During the course of an infection, the microorganism can pit a vast population of its species against an individual Homo sapiens. In response, the human body invests heavily in cells dedicated to defense, which collectively form the immune system.
