Page 509 - Review of Medical Microbiology and Immunology ( PDFDrive )

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mebooksfree.com mebooksfree.com mebooksfree.com AGE & THE IMMUNE RESPONSE mebooksfree.com
PART VII Immunology
498
Foreignness
In general, molecules recognized as “self” are not immuno-
Immunity is less than optimal at both ends of life (i.e., in
genic (i.e., we are tolerant to those self-molecules) (see
the newborn and the elderly). The reason for the relatively
Chapter 66). To be immunogenic, molecules must be rec-
poor immune response in newborns is unclear, but new-
ognized as “nonself” (i.e., foreign).
borns appear to have less effective T-cell function than do
Molecular Size
the transfer of maternal IgG across the placenta. Because
maternal antibody decays over time (little remains by 3–6
The most potent immunogens are proteins with high molec- adults. In newborns, antibodies are provided primarily by
months of age), the risk of infection in the child is high.
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ular weights (i.e., above 100,000). Generally, molecules with
Colostrum also contains antibodies, especially secretory
molecular weight below 10,000 are weakly immunogenic,
IgA, which can protect the newborn against various respi-
and very small ones (e.g., an amino acid) are nonimmuno-
genic. Certain small molecules (e.g., haptens) become
The fetus can mount an IgM response to certain anti-
immunogenic only when linked to a carrier protein.
gens (e.g., to Treponema pallidum, the cause of syphilis,
which can be acquired congenitally). IgG and IgA begin to
Chemical–Structural Complexity
be made shortly after birth. The response to protein anti-
gens is usually good; hence hepatitis B vaccine can be given
A certain amount of chemical complexity is required (e.g.,
at birth and poliovirus immunization can begin at 2
amino acid homopolymers are less immunogenic than het-
eropolymers containing two or three different amino acids).
age of 2 years respond poorly to polysaccharide antigens
unless they are conjugated to a carrier protein. For exam-
Antigenic Determinants (Epitopes) months of age. However, neonates and children under the
ple, the pneumococcal vaccine containing the unconju-
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Epitopes are small chemical groups on the antigen mole-
gated polysaccharides does not induce protective immunity
cule that can elicit and react with antibody. An antigen can
when given prior to 18 months of age, but the pneumococ-
have one or more determinants (epitopes). Most antigens
have many determinants (i.e., they are multivalent). In gen-
carrier protein is effective when given as early as 2 months
eral, a determinant is roughly five amino acids or sugars in
of age. This indicates the children under the age of two
size. The overall three-dimensional structure is the main
years do not mount a protective T-independent response
criterion of antigenic specificity.
(see Chapter 58, page 512).
In the elderly, immunity generally declines. There is a
Dosage, Route, and Timing of Antigen
reduced IgG response to certain antigens, fewer T cells, and
Administration
young, the frequency and severity of infections are high.
These factors also affect immunogenicity. In addition, the
The frequency of autoimmune diseases is also high in the
genetic constitution of the host (HLA genes) determines a reduced delayed hypersensitivity response. As in the very
elderly, possibly because of a decline in the number of regu-
whether a molecule is immunogenic. Different strains of
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mebooksfree.com mebooksfree.com mebooksfree.com SELF-ASSESSMENT QUESTIONS mebooksfree.com
latory T cells, which allows autoreactive T cells to prolifer-
the same species of animal may respond differently to the
ate and cause disease.
same antigen.
Adjuvants
Adjuvants enhance the immune response to an immuno-
1. Which one of the following is an attribute of the innate, rather
gen. They are chemically unrelated to the immunogen and
than the adaptive (acquired), arm of our host defenses?
differ from a carrier protein because the adjuvant is not
(A) Is highly specific in its response to bacteria
(B) Responds to viruses and fungi, but not bacteria
covalently bound to the immunogen, whereas the carrier
(C) Exhibits memory following exposure to bacteria
protein is. Adjuvants can act in a variety of ways; they can
(D) Is part of our host defense against bacteria but not against fungi
cause slow release of immunogen, thereby prolonging the
(E) Is as effective the first time it is exposed to bacteria as it is
stimulus; enhance uptake of immunogen by antigen-pre-
subsequent times
senting cells; and induce costimulatory molecules (“second 2. Regarding antibody-mediated immunity and cell-mediated
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signals”). (See Chapter 58 regarding costimulators.)
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immunity, which one of the following is the most accurate?
Another important mechanism of action of some adjuvants
(A) Antibody-mediated immunity helps prevent graft rejection.
is to stimulate Toll-like receptors (see pages 494 and 505)
on the surface of macrophages, which results in cytokine
(C) Antibody-mediated immunity protects against autoimmune
production that enhances the response of T cells and B cells
diseases.
to the immunogen (antigen). Some human vaccines con-
(D) Cell-mediated immunity neutralizes extracellular viruses.
tain adjuvants such as aluminum hydroxide or lipids.
(E) Cell-mediated immunity protects against fungal infections.
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