Classes of immunoglobulins Table 12.1
Name and Structure
Characteristics
Functions
IgG
IgA
IgM
IgD
About 80% of all antibodies in the blood. Also found in lymph
Protects against bacteria and viruses by
and the intestines. Only class of antibody to cross the pla-
enhancing phagocytosis, neutralizing
centa from mother to fetus, thereby conferring considerable
toxins, and triggering the complement
immune protection to newborns.
system.
About 10% to 15% of all antibodies in the blood. Found
mainly in sweat, tears, saliva, mucus, breast milk, and gastro-
intestinal secretions. Levels decrease during stress, lowering
resistance to infection.
Provides localized protection against bacte-
ria and viruses on mucous membranes.
About 5% to 10% of all antibodies in the blood. Also found
in lymph. First antibody class to be secreted by plasma cells
after an initial exposure to any antigen. In blood plasma, the
anti-A and anti-B antibodies of the ABO blood group, which
bind to A and B antigens during incompatible blood transfu-
sions, are also IgM antibodies (see Figure 10.8).
Activates complement and causes aggluti-
nation and lysis of microbes.
¥
About 0.2% of all antibodies in the blood. Also found in
Involved in activation of B cells.
lymph and on the surfaces of B cells as antigen receptors.
IgE
¥
Less than 0.1% of all antibodies in the blood. Also located on
Involved in allergic and hypersensitivity
mast cells and basophils.
reactions and provides protection against
parasitic worms.
The structure of antibodies allows them to be specif-
ic for a particular antigen. Antibodies belong to a larger
group of plasma proteins called immunoglobulins (Ig;
im-u-no-GLOB-u-lins). There are five classes of immuno-
globulins, each with different functions (Table 12.1).
Immune Response Time Speeds Up
After the Initial Exposure
During an initial infection, there are few lymphocytes
(that is, helper T cells, cytotoxic T cells, and B cells) with
the appropriate antigen receptor, so the
in itia l,
or
p rim a ry,
im m u n e response
may take several days or even weeks. Dur-
ing cell-mediated or antibody-mediated immunity, the
clonal selection process described earlier produces numer-
ous memory cells in each of those lymphocyte categories.
Because these memory cells live for long periods of time
(even decades), they can help your body respond to later
infections with the same antigens. In a subsequent expo-
sure to the same antigens, a
secondary im m u n e response
occurs
that is stronger and much more rapid (within hours or
days) (Figure 12.12). The presence of long-lasting cells
and antibodies gives your lymphatic system immunologi-
cal memory.
Im m unological m em ory • Figure 12.12
The stronger and more rapid reaction to subsequent antigen
exposures by memory cells can be seen in graphs of antibody
concentrations in the blood.
First exposure
Second exposure
Immune Reponses Help Protect the Body Against Disease
359
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