Determinants of Antigenicity
A number of factors have been identified that make a substance
immunogenic. Some of the important determinants of antige-nicity include:
1.
Molecular size
2.
Foreignness
3.
Chemical-structural complexity
4.
Stability
5.
Other factors
In general, protein molecules with large molecular weight are
highly antigenic. Substances with molecular weights of about 100,000 Da and
more are highly immunogenic, while substances with molecular weights of less
than 5000 Da are generally not immunogenic. This property has been exploited in
experimen-tal studies by using high molecular weight proteins like bovine gamma
globulin (MW 150,000 Da) to induce an immune reac-tion. Substances with low
molecular weight may be made anti-genic by adsorbing these on carrier
particles, such as bentonite, kaolin, and other inert particles.
To be immunogenic, a molecule must be recognized as nonself, i.e.,
foreign. The molecule is considered self or nonself by the immune system depending
on whether or not the molecule was exposed to the immune system during fetal
development.
Foreignness implies ability of the host to tolerate
self-antigens. Tolerance to self-antigens develops by contact with them in the
initial phases of the development of immune sys-tem, particularly during the
development of lymphocytes.
In general, the more distantly related two species are, the greater
the immunogenicity of a molecule from one species will be when exposed to the
other. For example, the bovine serum albumin is more immunogenic in a chicken
than in a goat. A graft from an unrelated human will be rejected within about 2
weeks unless immunosuppressive drugs are used, but a graft from a chimpanzee
will be rejected within hours even if drugs are used. In contrast, a kidney
graft from an identical twin will be accepted readily.
Proteins are the most potent immunogens
followed by polysaccha-rides. Nucleic acids and lipids are not efficient in
eliciting a good immune reaction, although they may act as haptens. Structural
complexity of a protein contributes to its immunogenicity. Chains of single
amino acids or single sugars are poorly immunogenic, but if different amino
acids or sugars are combined in the same molecule, the immunogenicity is
greatly enhanced.
In cell-mediated immunity, the response of T cells to the peptide
component of the proteins depends on how the pep-tide is recognized and
presented by the MHC cells. Therefore, the structure of protein plays an
important role in its immuno-genicity, especially in inducing cellular
immunity.
The lipid-specific antibodies are not easily produced; hence, they
do not play a major role in immunity. However, these antibodies have a role in
the measurement of certain lipid-based molecules and drugs. These antibodies
are produced first by treat-ing lipids with haptens and then conjugating with
suitable carrier molecules, such as the proteins (e.g., hemocyanin or bovine
serum albumin).
Highly stable and nondegradable substances (e.g., some plastics,
metals, or chains of D-amino acids) are not immunogenic.
This is because internalization, processing, and presentation by
antigen-presenting cells (APCs) are always essential to mount an immune
response. Therefore, very stable substances (such as silicon) have been
successful as nonimmunogenic materials for reconstructive surgeries, such as
breast implants.
On the other hand, if a substance is very unstable, it may break up
before an APC can be internalized, and hence become immunogenic. In addition,
large, insoluble complexes are more immunogenic than smaller, soluble ones.
This is because macrophages find it easier to phagocytose, degrade, and
pres-ent the insoluble complexes than the soluble complexes.
Biological system also plays an important role in determining the
immunological efficiency of an antigen. Some substances are immunogenic in one
individual but not in others (i.e., responders and nonresponders). This is due
to the fact that individuals may lack or have altered genes that code for the
receptors for antigen on B cells and T cells, or they may not have the
appropriate genes needed for the APC to present anti-gen to the helper T (TH)
cells.
The dose of antigen and the route by which it comes into contact
with the immune system also influence immunoge-nicity of the antigen. Very low
doses of antigen do not stimu-late immune response, either because too few
lymphocytes are contacted or because a nonresponsive state is elic-ited.
Conversely, an extremely high dose also fails to elicit tolerance.
Repeated administration of antigens (booster doses) may be required
to enhance immune response of the host to certain antigens. This is
particularly important in case of vac-cines where a prerequisite immune level
needs to be attained. Hence the booster doses of vaccines, such as DPT
(Diphtheria, Pertussis, Tetanus), DT (Diphtheria, Tetanus), etc., are given to
ensure good protective levels of antibodies. Generally, antigens are
administered by the parenteral route to produce good level of antibodies. The
antigens can be given by (a)
intravenous, (b) subcutaneous, (c) intradermal, (d) intramuscular, (e)
intra-peritoneal, and (f) mucosal
routes. Usually, the subcutaneous route of administration proves to be better
than intravenous routes at eliciting an immune response.
Adjuvants are the substances that when mixed with an anti-gen and
injected with it boost the immunogenicity of the anti-gen. Adjuvants increase
both the strength and the duration of immune response. Adjuvants boost
immunogenicity of anti-gens in several ways:
Adjuvants like aluminum
potassium sulfate (alum) and Freund’s water-in-oil adjuvant prolong the
persistence of antigen by forming a depot at the injection site. Alum
precipitates the antigen
and releases it a little at a time. The water-in-oil emulsion forms small
droplets with the antigen and also releases these slowly over time.
Freund’s
complete adjuvant contains, in addition to the emulsifying factors, heat-killed
mycobacteria. The bacterial components activate macrophages and increase both
the production of IL-1 and the level of B7 membrane molecules, which enhances
the immune response. The increased expression of class II MHC increases the
ability of APC to present antigen to TH cells. B7 molecules on the APC bind to CD28, a
cell-surface protein on TH cells, triggering costimulation, an
enhancement of the T-cell immune response.
Some adjuvants, like
synthetic polyribonucleotides and bacterial lipopolysaccharides, stimulate
nonspecific lym-phocyte proliferation and bring about their action.
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