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Immune Defense against Bacterial Pathogens: Adaptive or Acquired Immunity (page 1)
(This chapter has 6 pages)
© Kenneth Todar, PhD
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Human Blood Cells;
erythrocytes, neutrophils, eosinophils, basophils,
monocytes and lymphocytes. The white blood cells are central players in
the immune responses.
Adaptive Immunity
Innate immunity, by itself, may not be sufficient
to
protect a host against an invading pathogen or to prevent disease from
occurring. However, if innate immunity fails, the organism may yet be
detected
and attacked by the mechanisms of adaptive immunity.
The innate and adaptive immune responses both function to protect
against invading organisms, but they differ in a number of ways. (1)
The innate immune system is constitutively present and reacts
immediately to infection. The adaptive immune response to
an invading organism takes some time to develop. (2) The
innate immune system is not specific in its response and reacts equally
well to a variety of organisms, whereas the adaptive immune system is
antigen-specific and reacts only with the organism that induced the
response. (3) The adaptive immune system exhibits immunological
memory. It "remembers" that it has encountered an invading organism
(antigen) and
reacts more rapidly on subsequent exposure to the same organism. The
innate immune system does not possess a memory.
Antigens
An antigen (Ag) is
a foreign substance (i.e., not part of the animal's tissues) of
relatively high molecular weight (>12,000
daltons) that induces a specific immunological response in the form of
AMI or CMI or both. Because of their complex
macromolecular
structure, a single microorganism consists of multiple antigens (e.g.
surface
structures such as cell wall components, fimbriae, flagella, etc., or
extracellular
proteins, such as toxins and extracellular enzymes). The
coat proteins and some of the envelope proteins of animal viruses are
also antigenic. The animal host is able to respond specifically to
each and
every antigen to come into contact with the components of the
immunological
system.
Adaptive immunity is a function of the immunological system
(Figure 1). The immunological system is able to recognize specific
antigens and react in such a way that the host generates
antibody-mediated immunity
(AMI),
cell-mediated
immunity (CMI), or both. AMI and CMI are the
two
great arms of the immunological response discussed below.
Active vs passive immunity
Although adaptive immunity develops in an animal which is undergoing a
specific immunological response to an antigen, the immune cells and
factors generated can be shared among two or more animals. Hence,
adaptive immunity can be acquired by an animal in two ways: active immunity and passive immunity.
1. In the case of active immunity, the animal undergoes an
immunological
response to an antigen and produces the cells and factors responsible
for the
immunity,
i.e., the animal produces its own antibodies and/or immuno-reactive
lymphocytes.
Active immunity can persist a long time in the animal, up to many years
in
humans.
2. Passive immunity is the acquisition by an animal of
immune
factors
which
were produced in another animal, i.e., the host receives antibodies
and/or
immuno-reactive lymphocytes originally produced during an active
response
in another animal. Passive immunity is typically short-lived and
usually
persists for only a few weeks or months.
Furthermore, either active or passive immunity may be
acquired
by natural means (e.g. self production of antibodies during
infection or transfer of antibodies from mother to offspring) or by artificial
means (i.e.,
vaccination
and other immunization procedures). Some familiar examples of active
and
passive
immunity are given in the table below.
Table 1. Examples
of Active and
Passive
Immunity
| Type of Immunity |
How Acquired by Host |
Examples |
| Active Immunity |
As a result of exposure to an infectious
agent or one of its
products
(antigens) |
Natural: Antibodies are produced by the host
in response to
the
infectious agent
itself (e.g. recovery from the disease).
Artificial: immunization
(vaccination) with some product derived from the infectious agent (e.g.
toxoid, killed cells, structural components of cells, inactivated
or attenuated viruses,
etc.).
|
| Passive Immunity |
As a result of the acquisition of antibodies
which have been
produced
in another animal (by active means) or derived from cells grown in
tissue
culture (e.g. monoclonal antibodies) |
Natural: Transplacental transfer of
antibodies
from
mother to fetus; transfer of antibodies from mother to infant in milk
by
nursing.
Artificial: Injection of immune serum from an individual previously
immunized or
recovered from disease, e.g. hepatitis; injection of serum from an
animal
hyperimmunized with tetanus toxoid. |
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