Molecular Immunology
The ability of vertebrate animals to resist and overcome
disease caused by infectious agents and toxic proteins
is provided by the immune system. The immune system
comprises specialized tissues, cells, highly specific pro-
teins (e.g., antibodies and receptors), and unique genetic
mechanisms for creating the many different defenses that
the organism requires for combating infectious diseases.
Table 35-1 provides a glossary and cross-references to ta-
bles and figures that relate these components and their
functions to the molecules of the immune system. The
cells and molecules composing the immune system are
designed to attack and destroy infectious and foreign sub-
stances, but do not under normal circumstances attack the
host organism’s tissues, cells, or molecules. This ability
to discriminate between self and nonself is an essential
property of the immune recognition system.
Immunity is obtained through reactions of cells and
molecules that are components of two distinguishable im-
The term immunity, which is derived from the Greek
im m u n ita s
, means
exemption from service or duty to state; when extended to the organism, it
means exemption from disease. Abbreviations: Ab, antibody; Ag, antigen;
MAb, monoclonal antibody; TcR, T-cell receptor; Th, T helper cell; Tc, cy-
totoxic T cell; CH, constant region, heavy chain; CL, constant region, light
chain; VH, variable region, heavy chain; VL, variable region, light chain;
CDR, complementarity-determining region; CD, cluster of differentiation;
IL, interleukin; MAC, membrane attack complex Of, complement com-
ponent (numbered); Fab, antibody fragment, single arm; F(ab)', antibody
fragment, two arms; Fc, antibody fragment, heavy chains; ADCC antibody-
dependent, cell-mediated cytoxicity.
mune response systems. The first, innate immunity, is the
simpler of the two systems and remains essentially un-
changed upon repeated exposure to an infectious agent or
foreign substance. In contrast, the second system, acquired
immunity, is enhanced upon repeated exposure to infec-
tious agents. The “memory” capacity of acquired immu-
nity enables the immune system to respond more rapidly
and more extensively when an infectious agent or foreign
substance is encountered a second time and is the basis
for resistance to organisms previously encountered and
explains the effectiveness of vaccination.
Innate Immunity
The first line of defense against infectious agents is pro-
vided by the innate immunity of the organism. This de-
fense is the result of physical barriers such as skin, the
cells that line the gastrointestinal and genitourinary tracts,
and the mucus that these cells secrete.
secretions act to move infectious agents
away from their sites of attachment and out of the
body. Infections from bacteria are also opposed by the
enzyme lysozyme, which is capable of degrading the
carbohydrate structures of the bacterial cell walls. One
particular type of antibody molecule, IgA (see below),
is present in the mucous secretions and participates
in this first line of defense against invading organisms.
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