Immunity, or adaptive specific defenses, also known as the third line of defense, is defined as resistance to specific pathogens or their toxins and metabolic byproducts.
Adaptive
(Specific) Defenses
Immunity, or adaptive
specific defenses, also known as the third
line of defense, is defined as resistance to specific pathogens or their
toxins and metabolic byproducts. Adaptive immune responses are carried out by
lympho-cytes and macrophages that recognize and remember certain foreign
molecules. As a fetus develops, cells learn to recognize proteins and large
molecules as being “self.” The lymphatic system, as it develops, responds to
“non-self” or foreign antigens and not, if the system is normal, the “self”
antigens. Immunity is therefore “triggered” by initial exposures to antigens.
After exposure, it can effec-tively protect the body. When immunity is disabled
or fails to protect the body, serious diseases such as AIDS or cancer can
develop.
A normal immune system protects the body against infectious
agents and abnormal cells. The adap-tive immune system can greatly increase the
inflam-matory response, and is responsible for the majority of complement
activation.
Antigens
include proteins, polysaccharides, glyco-proteins, and glycolipids
that are commonly found on cell surfaces. Antigens can mobilize adaptive
defenses and cause an immune response. All adap-tive immune responses
ultimately target antigens, which are usually large and complex molecules not
normally present in the body. Antigens may be natu-ral or synthetic in nature
and complete or incomplete. Complete
antigens have both immunogenicity andreactivity. Immunogenicity is the ability to
stimu-late certain lymphocytes to multiply. Reactivity is the ability to react with activated
lymphocytes and antibodies that are released via immunogenic reac-tions.
Proteins are the strongest types of antigens. Microorganisms and grains of
pollen are immu-nogenic because their surfaces hold many different types of
foreign macromolecules.
A small molecule, or incomplete antigen, which cannot stimulate an immune
response by itself, is known as a hapten. It is
found in certain drugs such as penicillin, in dust particles, animal dander,
poison ivy, detergents, cosmetics, and in various household and industrial
chemicals. Haptens usually combine with larger, more complex molecules to
elicit an immune response.
The size and complexity of a molecule determines how it will
be able to act as an antigen. The only parts of an antigen that are immunogenic
are its antigenic determinants.
Antibodies or lymphocyte recep-tors bind to antigenic determinants similarly to
how enzymes bind to substrates. The majority of naturally occurring antigens
have various antigenic determi-nants located on their surfaces. Some antigenic
deter-minants are more potent than others, in relation to the immune response
they cause. Different lympho-cytes react to different antigenic determinants.
This means that a single antigen can mobilize several types of lymphocytes,
causing the formation of many types of antibodies.
Larger proteins have hundreds of antigenic deter-minants,
all with different chemical makeups. This explains their high reactivity and
immunogenicity. However, there is little or no immunogenicity in large but
simple molecules, such as plastics. These mol-ecules have many identical and
regularly repeating units. Substances such as these plastics can be used to
make artificial implants, since the body does not sense them as being
“foreign,” and therefore, it does not reject the implants.
All body cells have many different protein mole-cules on
their surfaces. In a normal immune system, self-antigens are not foreign or
antigenic to thebody, but are extremely antigenic to other people. This concept
is at the core of transfusion reactions and graft rejections. The MHC proteins
are among the cell sur-face proteins that identify each cell as “self.” Genes
of the MHC code for these proteins. There are millions of combinations of such
genes that are possible. There-fore, it is not likely that any two individuals,
except for identical twins, will have the same MHC proteins. Each of these
proteins has a deep groove holding a peptide that is either a self-antigen or a
foreign anti-gen. T lymphocytes only bind antigens presented to them on MHC
proteins.
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