TABLE 1.

Components of the Immune System

Component	Function
B lymphocytes	Production of antibodies: IgG: defends against bacteria, viruses, and toxins; activates complement; main antibody in the plasma IgA: called the secretory Ig; defends against bacteria and viruses on mucosal surfaces IgM: first antibody produced by immature B lymphocytes; reacts with antigens on some red blood cells following incompatible blood transfusions; activates complement IgD: B cell activation; exact function unknown IgE: promotes inflammation and allergic reactions
T lymphocytes	Direct attack of viruses, fungi, or transplanted tissue Regulation of immune system Types of T lymphocytes: Cytotoxic: directly binds to target and kills it Helper: enhances function of B lymphocytes; assist cytotoxic T cells in attack on foreign substance Regulatory: suppress other T lymphocytes and keeps lymphocyte system in check
Natural killer cells	Killing of virus-infected cells Secretion of chemicals to enhance inflammation
Phagocytes	Ingestion and elimination of microorganisms Types of phagocytes: Neutrophils/granulocytes Monocytes Macrophages
Complement system	Component of innate immunity Types of complement proteins: C1 through C9 C1 inhibitor Factor B Factor D

Sources: Adapted from Blaese 2007 and Shier 2010.