Page 504 - Review of Medical Microbiology and Immunology ( PDFDrive )

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CHAPTER 57 Immunity
Virus-infected cell

Class I MHC
protein
Perforins
Virus
Viral epitope
TCR

CD8 IL-2 Macrophage
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cell
IgM
IL-4, IL-5
Class II MHC protein
Viral epitope
TCR
cell
MHC protein
IL-2
FIGURE 57–3
Induction of cell-mediated immunity and antibody against a viral infection. Right: Virus released by an infected cell is
ingested and processed by an antigen-presenting cell (APC) (e.g., a macrophage). The viral epitope is presented in association with a class II
major histocompatibility complex (MHC) protein to the virus-specific T-cell receptor (TCR) on the CD4 cell. The macrophage makes interleukin
(IL)-1, which helps activate the CD4 cell. The activated CD4 cell makes interleukins (e.g., IL-2, which activates the CD8 cell to attack the virus-
infected cell, and IL-4 and IL-5, which activate the B cell to produce antibody). The specificity of the cytotoxic response mounted by the CD8 cell
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is provided by its TCR, which recognizes the viral epitope presented by the virus-infected cell in association with a class I MHC protein. Left:
Virus released by an infected cell interacts with the antigen receptor (IgM monomer) specific for that virus located on the surface of a B cell. The
virus is internalized, and the viral proteins are broken down into small peptides. B cells (as well as macrophages) can present viral epitopes in
association with class II MHC proteins and activate CD4 cells. The CD4-positive helper cell produces IL-4 and IL-5, which induce the B cell to
differentiate into a plasma cell that produces antibody specifically against this virus.

recognize antigens with their surface IgM that acts as an
antigen receptor, and (2) they present epitopes to helper T
cells), and certain proteins (e.g., the complement cascade
cells in association with class II MHC proteins. Note that
and interferons) and involves processes such as phagocyto-
the IgM antigen receptor on the B cell can recognize not
not improve after exposure to the organism, in contrast to
only foreign proteins but also carbohydrates, lipids, DNA,
acquired immunity, which does. In addition, innate
RNA, and other types of molecules. The class II MHC pro-
immune processes have no memory, whereas acquired
teins of the B cell, however, can only present peptide frag- sis and inflammation (Table 57–4). Innate immunity does
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immunity is characterized by long-term memory.
ments to the helper T cells. This distinction will become
Note that the innate arm of our host defenses performs
important when haptens are discussed later in this chapter.
two major functions: killing invading microbes and activat-
It is this remarkable ability of the IgM antigen receptor on
the B cell to bind to an incredibly broad range of molecules
that enables B cells to produce antibodies against virtually
innate arm, such as neutrophils, only kill microbes, whereas
every molecule known. How the B cell generates such a
others, such as macrophages and dendritic cells, perform
both functions (i.e., they kill microbes and present antigen to
diverse array of antibodies is described on page 528.
helper T cells, which activates adaptive immune processes).
Although innate immunity is often successful in elimi-
INNATE & ADAPTIVE IMMUNITY
Our immune host defenses can be divided into two major
sufficient for human survival. This conclusion is based on
categories: innate (natural) and adaptive (acquired). The
the observation that children with severe combined immu-
features of these two important components of our host nating microbes and preventing infectious diseases, it is not
nodeficiency disease (SCID), who have intact innate
defenses are compared in Table 57–2.
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immunity but no adaptive immunity, suffer from repeated,
life-threatening infections.
1. Innate Immunity
Properties of Innate Immunity
by Innate Immunity
Innate immunity is resistance that exists prior to exposure
Several components of the innate arm recognize what is
to the microbe (antigen). It is nonspecific and includes
foreign by detecting certain carbohydrates or lipids on the
host defenses such as barriers to infectious agents (e.g., skin
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