Definition

The immune system is the body's defense team, with barriers like skin to keep germs out and white blood cells that hunt down and destroy germs that get in, which is why a cut turns red and puffy as white blood cells rush to the spot, and it remembers germs it has fought before, which is why you usually get chickenpox only once. More precisely, the innate immune system responds quickly and generally with barriers, inflammation, and phagocytic white blood cells such as macrophages, while the adaptive immune system responds more slowly but specifically, with B cells making antibodies against a pathogen's antigens, T cells destroying infected cells, and memory cells staying behind so a second exposure is defeated before illness develops, cutting the antibody response from one to two weeks on first exposure down to a few days on the next. At the cellular level, innate defenses combine physical barriers, complement, phagocytes, natural killer cells, and pattern-recognition receptors with adaptive immunity based on B and T lymphocytes, each carrying a unique receptor generated by V(D)J recombination, with clonal selection expanding only the lymphocytes whose receptors bind an antigen presented by MHC molecules; B cells differentiate into antibody-secreting plasma cells, cytotoxic T cells kill infected cells, helper T cells coordinate the response through cytokines, and regulatory mechanisms and self-tolerance normally prevent autoimmunity.