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
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
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.
