Download Specificity of primary and secondary responses

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Immunology
Immune response
Difference between Primary and Secondary Immune Response!
Imagine a person is never exposed to a particular immunogen. For the first time in his life one
antigen enters into his body (this event is called Priming). This leads to a relatively week,
short-lived immune responses called Primary immune responses. The primary immune
responses can be divisible into four phases (lag phase, exponential phase, steady state phase,
and declining phase) (Fig. 7.1).
a. The lag (latent) phase is the period from the initial exposure of immunogen to the time of
detection of antibodies (In humans the average time of lag phase is about one week). During
this lag phase specific T cells and B cells are activated by their contact with immunogen.
b. The exponential phase is the period during which there is a rapid increase in antibody levels
due to secretion of antibodies by many plasma cells.
c. After the exponential phase the antibody level remains relatively at a constant level because
the secretion and degradation of antibodies occur almost at equal rates. This phase is called
steady state phase (plateau phase).
d. Afterwards the antibody level gradually declines (declining phase) because new plasma
cells are no longer produced and the existing plasma cells are dying. This generally indicates
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Immunology
that the immunogen has been eliminated from the body and consequently there is no stimulus
for continued antibody production.
When a similar antigen enters the host for the second and subsequent times, the immune
responses induced are called secondary immune responses. During secondary immune
responses the lag period is shortened and antibody level reaches a very high steady state level
within few days. Since the secondary immune responses are induced rapidly (within a short
time after the entry of the antigen) to greater levels, the antigen is eliminated before it can
cause damage or disease. More over the antibodies remain in circulation for a longer period.
The secondary immune responses are also called anamnestic immune responses. (Amnesia =
forget; anamnesis = don’t forget, remember). Since specific memory T and B cells for the
immunogen have already been produced during the primary response, the lag phase is shorter
when compared to the primary immune response.
Comparison between Primary and Secondary Immune Response
Primary Immune Response
Secondary Immune Response
After initial exposure to a foreign antigen,
there is a lag phase where B cells are
differentiating into plasma cells, but not yet
producing antibodies. Antibody generation
can take anything from 2 days to several
months.
If a previously encountered antigen enters
the body again, a few days up to several
years later, a secondary immune response
develops. This time, the lag phase is greatly
reduced, to about 3-4 days.
Low quantities of antibodies are normally
secreted.
High levels of antibodies are produced.
After a while, the amount of antibody
decreases to minimal levels.
Antibody levels remain elevated for longer.
Antibodies are mostly IgM, but some IgG
antibodies are produced.
IgGs are the main antibody secreted, with
some small amounts of IgM sometimes.
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Immunology
GENERAL CHARACTERISTICS OF THE ANTIBODY RESPONSE
Self / non-self-discrimination
One characteristic feature of the specific immune system is that it normally distinguishes
between self and non-self and only reacts against non-self.
Memory
A second feature of the specific immune response is that it demonstrates memory. The
immune system "remembers" if it has seen an antigen before and it reacts to secondary
exposures to an antigen in a manner different than after a primary exposure. Generally only an
exposure to the same antigen will illicit this memory response.
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Immunology
Specificity
A third characteristic feature of the specific immune system is that there is a high degree of
specificity in its reactions. A response to a particular antigen is specific for that antigen or a few
closely related antigens.
Fate of the immunogen
1-Clearance after primary injection.
The kinetics of antigen clearance from the body after a primary administration is depicted in
Figure 1.
 Equilibrium phase
The first phase is called the equilibrium or equilibration phase. During this time the
antigen equilibrates between the vascular and extravascular compartments by diffusion.
This is normally a rapid process. Since particulate antigens don't diffuse, they do not show
this phase.
 Catabolic decay phase
In this phase the host's cells and enzymes metabolize the antigen. Most of the antigen is
taken up by macrophages and other phagocytic cells. The duration will depend upon the
immunogen and the host.
 Immune elimination phase
In this phase, newly synthesized antibody combines with the antigen producing
antigen/antibody complexes which are phagocytosed and degraded. Antibody appears in
the serum only after the immune elimination phase is over.
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Immunology
2- Clearance after secondary injection
If there is circulating antibody in the serum, injection of the antigen for a second time results
in a rapid immune elimination. If the is no circulating antibody then injection of the antigen for
a second time results in all three phases but the onset of the immune elimination phase is
accelerated.
Specificity of primary and secondary responses
Antibody elicited in response to an antigen is specific for that antigen, although it may also
cross react with other antigens which are structurally similar to the eliciting antigen. In general
secondary responses are only elicited by the same antigen used in the primary response.
However, in some instances a closely related antigen may produce a secondary response, but
this is a rare exception.