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UNIT IV
Infection, Inflammation, and 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 adap-
tive 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.
Understanding
Innate and Adaptive Immunity
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 com-
ponents of innate immunity include epithelial cells, which
block the entry of infectious agents and secrete antimicro-
bial enzymes, proteins, and peptides; phagocytic neutrophils
and macrophages, which engulf and digest microbes; natural
killer (NK) cells, which kill intracellular microbes and for-
eign 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 distin-
guish between agents.
C5b
C6,C7,C8,C9
Lysis of
microbe
Complement
Epithelial barriers
Microbe
NK cells
Phagocytosis
Monocyte/
macrophage Neutrophil
Cell death
Membrane
attack
complex
