Page 107 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Immune System At first contact with a virus (e.g., measles
virus), the nonspecific immune system usually
Fundamental Principles cannot prevent the viral proliferation and the
The body has nonspecific (innate) immune development of the measles. The specific im-
defenses linked with specific (acquired) im- mune system, with its T-killer cells (! B2) and
mune defenses that counteract bacteria, Ig (first IgM, then IgG; ! C3), responds slowly:
viruses, fungi, parasites and foreign (non-self) primary immune response or sensitization.
macromolecules. They all function as antigens, Once activated, it can eliminate the pathogen,
i.e., substances that stimulate the specific im- i.e., the individual recovers from the measles.
mune system, resulting in the activation of an- Secondary immune response: When infected a
tigen-specific T lymphocytes (T cells) and B second time, specific IgG is produced much
lymphocytes (B cells). In the process, B lym- more rapidly. The virus is quickly eliminated,
phocytes differentiate into plasma cells that and the disease does not develop a second
secrete antigen-specific antibodies (immuno- time. This type of protection against infectious
globulins, Ig) (! C). Ig function to neutralize disease is called immunity. It can be achieved
and opsonize antigens and to activate the com- by vaccinating the individual with a specific
plement system (! p. 96). These mechanisms antigen (active immunization). Passive im-
ensure that the respective antigen is specifi- munization can be achieved by administering
cally recognized, then eliminated by relatively ready-made Ig (immune serum).
Blood nonspecific means. Some of the T and B cells Nonspecific Immunity
have an immunologic memory.
Precursor lymphocytes without an antigen-
4 binding receptor are preprocessed within the Lysozyme and complement factors dissolved in
plasma (! A1) as well as natural killer cells (NK
thymus (T) or bone marrow (B). These organs cells) and phagocytes, especially neutrophils
produce up to 10 monospecific T or B cells, and macrophages that arise from monocytes
8
each of which is directed against a specific an- that migrate into the tissues (! A2) play an
tigen. Naive T and B cells which have not pre- important role in nonspecific immunity.
viously encountered antigen circulate through Neutrophils, monocytes, and eosinophils cir-
the body (blood ! peripheral lymphatic tissue culate throughout the body. They have
! lymph ! blood) and undergo clonal expan- chemokine receptors (e.g., CXCR1 and 2 for IL-
sion and selection after contact with its 8) and are attracted by various chemokines
specific antigen (usually in lymphatic tissue). (e.g., IL-8) to the sites where microorganisms
The lymphocyte then begins to divide rapidly, have invaded (chemotaxis). These cells are able
producing numerous monospecific daughter to migrate. With the aid of selectins, they dock
cells. The progeny differentiates into plasma onto the endothelium (margination), pene-
cells or “armed” T cells that initiate the elimi- trate the endothelium (diapedesis), and engulf
nation of the antigen. and damage the microorganism with the aid of
lysozyme, oxidants such as H 2O 2, oxygen radi-
Clonal deletion is a mechanism for eliminating – 1
lymphocytes with receptors directed against auto- cals (O 2 , OH·, O 2), and nitric oxide (NO). This is
logous (self) tissue. After first contact with their followed by digestion (lysis) of the microor-
specific self-antigen, these lymphocytes are elimi- ganism with the aid of lysosomal enzymes. If
nated during the early stages of development in the the antigen (parasitic worm, etc.) is too large
thymus or bone marrow. Clonal deletion results in for digestion, other substances involved in
central immunologic tolerance. The ability of the nonspecific immunity (e.g., proteases and cy-
immune system to distinguish between endogenous totoxic proteins) are also exocytosed by these
and foreign antigens is called self/nonself recogni- cells.
tion. This occurs around the time of birth. All sub-
stances encountered by that time are recognized as Reducing enzymes such as catalase and superoxide
endogenous (self); others are identified as foreign dismutase usually keep the oxidant concentration
(nonself). The inability to distinguish self from non- low. This is often discontinued, especially when mac-
self results in autoimmune disease. rophages are activated (! below and B3), to fully ex-
94 ploit the bactericidal effect of the oxidants. However,

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