TABLE 2.

Key Components of the Immune System

Component	Functions
B lymphocytes	Principal effectors of humoral immunity in adaptive immune system Produce antibodies (immunoglobulins): - IgA: secreted from exocrine glands; defends against bacteria and viruses - IgD: activates B cells; present on the surfaces of most B lymphocytes - IgE: secreted from exocrine glands; promotes inflammation and allergic reactions - IgG: found in plasma and tissue fluid; defends against bacteria, viruses, and toxins; activates complement; crosses placenta, passing immunity from mother to newborn - IgM: found in plasma; first antibody formed in response to infection; reacts with antigens on red blood cells after incompatible blood transfusions; activates complement
T lymphocytes	Principal effectors of cellular immunity in adaptive immune system Directly attack foreign antigens, such as viruses, fungi, and transplanted tissue Regulate the immune system Types of T lymphocytes: - Killer (cytotoxic): directly bind to target and kill it - Helper: assist B cells in producing antibody; assist cytotoxic T cells in attacking foreign substances - Regulatory: suppress other T lymphocytes and keep lymphocyte system in check
Natural killer cells	Components of innate immunity Kill virus-infected cells Secrete chemicals that enhance inflammation
Phagocytes	Components of innate immunity Ingest and kill microorganisms Types of phagocytes: - Neutrophils/granulocytes - Monocytes - Macrophages
Complement system	Component of innate immunity Composed of 30 proteins that defend against infection and produce inflammation Types of complement proteins: - C1 through C9 - Factor B - Factor D

Sources: Adapted from Blaese 2007, Haynes 2008, Shier 2010.