Immunochemistry WORLD OF MICROBIOLOGY AND IMMUNOLOGY292•
See alsoAntibody and antigen; Antibody formation and kinet-
ics; Immunity, active, passive and delayed; Immunity, cell
mediated; Immunity, humoral regulationIImmunochemistryMMUNOCHEMISTRY
Immunochemistry is the study of the chemistry of immune
responses.
An immune response is a reaction caused by the inva-
sion of the body by an antigen. An antigen is a foreign sub-
stance that enters the body and stimulates various defensive
responses. The cells mainly involved in this response are
macrophages and T and B lymphocytes. A macrophage is a
large, modified white blood cell. Before an antigen can stimu-
late an immune response, it must first interact with a
macrophage. The macrophage engulfs the antigen and trans-
ports it to the surface of the lymphocytes. The macrophage (or
neutrophil) is attracted to the antigen by chemicals that the
antigen releases. The macrophage recognizes these chemicals
as alien to the host body. The local cells around the infection
will also release chemicals to attract the macrophages; this is
a process known as chemotaxis. These chemicals are a
response to the infection. This process of engulfing the foreign
body is called phagocytosis, and it leads directly to painful
swelling and inflammationof the infected area.
Lymphocytes are also cells that have been derived from
white blood cells (leucocytes). Lymphocytes are found in
lymph nodes, the spleen, the thymus, bone marrow, and circu-
lating in the blood plasma. Those lymphocytes that mature
inside mammalian bone marrow are called B cells. Once B
cells have come into contact with an antigen, they proliferate
and differentiate into antibodysecreting cells. An antibody is
any protein that is released in the body in direct response to
infection by an antigen. Those lymphocytes that are formed
inside the thymus are called T lymphocytes or T cells. After
contact with an antigen, T cells secrete lymphokines—a group
of proteins that do not interact with the antigens themselves,
instead they stimulate the activity of other cells. Lymphokines
are able to gather uncommitted T cells to the site of infection.
They are also responsible for keeping T cells and macrophages
at the site of infection. Lymphokines also amplify the number
of activated T cells, stimulate the production of more lym-
phokines, and kill infected cells. There are several types of T
cells. These other types include T helper cells that help B cells
mature into antibody-secreting cells, T suppresser cells that
halt the action of B and T cells, T cytotoxic cells that attack
infected or abnormal cells, and T delayed hypersensitivity
cells that react to any problems caused by the initial infection
once it has disappeared. This latter group of cells are long
lived and will rapidly attack any remaining antigens that have
not been destroyed in the major first stages of infection.
Once the antibodies are released by the B and T cells,
they interact with the antigen to attempt to neutralize it. Some
antibodies act by causing the antigens to stick together; this is a
process known as agglutination. Antibodies may also cause the
antigens to fall apart, a process known as cell lysis. Lysis is
caused by enzymesknown as lytic enzymes that are secreted bythe antibodies. Once an antigen has been lysed, the remains of
the antigen are removed by phagocytosis. Some antigens are
still able to elicit a response even if only a small part of the anti-
gen remains intact. Sometimes the same antibody will cause
agglutination and then lysis. Some antibodies are antitoxins,
which directly neutralize any toxins secreted by the antigens.
There are several different forms of antibody that carry out this
process depending upon the type of toxin that is produced.
Once antibodies have been produced for a particular
antigen they tend to remain in the body. This provides immu-
nity. Sometimes immunity is long term and once exposed to a
disease we will never catch the disease again. At other times,
immunity may only be short lived. The process of active
immunity is when the body produces its own antibodies to
confer immunity. Active immunity occurs after an initial expo-
sure to the antigen. Passive immunity is where antibodies are
passed form mother to child through the placenta. This form of
immunity is short lived. Artificial immunity can be conferred
by the action of immunization. With immunization, a vaccine
is injected into the body. The vaccine may be a small quantity
of antigen, it may be a related antigen that causes a less seri-
ous form of the disease, it may be a fragment of the antigen,
or it may be the whole antigen after it has been inactivated. If
a fragment of antigen is used as a vaccine, it must be sufficient
to elicit an appropriate response from the body. Quite often
viral coat proteins are used for this. The first vaccine was
developed by Edward Jenner(1749–1823) in 1796 to inocu-
late against smallpox. Jenner used the mild disease cowpoxto
confer immunity for the potentially fatal but biochemically
similar smallpox.
Within the blood there are a group of blood serum pro-
teins called complement. These proteins become activated by
antigen antibody reactions. Immunoglobulin is an antibody
secreted by lymphoid cells called plasma cells. Immuno-
globulinsare made of two long polypeptide chains and two
short polypeptide chains. These chains are bound together in a
Y-shaped arrangement, with the short chains forming the inner
parts of the Y. Each arm of the Y has specific antigen binding
properties. There are five different classes of immunoglobulin
that are based on their antigen-binding properties. Different
classes of immunoglobulins come into play at different stages
of infection. Immunoglobulins have specific binding sites
with antigens.
One class of compounds in animals has antigens that
can be problematical. This is the group called the histocom-
patibilitycomplex. This is the group of usually surface pro-
teins that are responsible for rejections and incompatibilities
in organ transplants. These antigens are genetically encoded
and they are present on the surface of cells. If the cells or tis-
sues are transferred from one organism to another or the body
does not recognize the antigens, it will elicit a response to try
to rid the body of the foreign tissue. A body is not interested
where foreign proteins come from. It is interested in the fact
that they are there when they should not be. Even if an organ
is human in origin, it must be genetically similar to the host
body or it will be rejected. Because an organ is much larger
than a small infection of an antigen when it elicits an immune
response, it can be a greater problem. With an organ trans-womi_I 5/6/03 3:23 PM Page 292