Innate and Adaptive Immunity
The body’s defense against microbes is mediated by two types of immunity: (1) innate immunity and (2) adaptive immunity. Both types of immunity are members of an integrated system in which numerous cells and molecules function cooperatively to protect the body against foreign invaders. The innate immune system stimulates adaptive immunity and influences the nature of the adaptive immune responses to make them more effective. Although they use different mechanisms of pathogen recognition, both types of immunity use many of the same effector mechanisms, including destruction of the pathogen by phagocytosis and the complement system.
Innate immunity (also called natural immunity) consists of the cellular and biochemical defenses that are in place before an encounter with an infectious agent and provide rapid protection against infection. The major effector components of innate immunity include epithelial cells, which block the entry of infectious agents and secrete antimicrobial enzymes, proteins, and peptides; phagocytic neutrophils and macrophages, which engulf and digest microbes; natural killer (NK) cells, which kill intracellular microbes and foreign agents; and the complement system, which amplifies the inflammatory response and uses the membrane attack response to lyse microbes. The cells of the innate immune system also produce chemical messengers that stimulate and influence the adaptive immune response.
The innate immune system uses pattern recognition receptors that recognize microbial structures (e.g., sugars, lipid molecules, proteins) that are shared by microbes and are often necessary for their survival, but are not present on human cells. Thus, the innate immune system is able to dis- tinguish between self and nonself, but is unable to distinguish between agents.
Adaptive immunity (also called acquired immunity) refers to immunity that is acquired through previous exposure to infectious and other foreign agents. A defining characteristic of adaptive immunity is the ability not only to distinguish self from nonself but to recognize and destroy specific foreign agents based on their distinct antigenic properties. The components of the adaptive immune system are the T and B lymphocytes and their products. There are two types of adaptive immune responses, humoral and cell-mediated immunity, that function to eliminate different types of microbes.
Humoral immunity is mediated by the B lymphocytes (B cells) and is the principal defense against extracellular microbes and their toxins. The B cells differentiate into antibody-secreting plasma cells. The circulating antibodies then interact with and destroy the microbes that are present in the blood or mucosal surfaces.
Cell-mediated, or cellular, immunity is mediated by the cytotoxic T lymphocytes (T cells) and functions in the elimination of intracellular pathogens (e.g., viruses). T cells develop receptors that recognize the viral peptides displayed on the surface of infected cells and then signal destruction of the infected cells.