WORLD OF MICROBIOLOGY AND IMMUNOLOGY Antibody and antigen27•
complete the job by engulfing the bacteria, destroying them,
and then removing them from the body.
There are five different antibody types, each one having
a different Y-shaped configuration and function. They are the
Ig G, A, M, D, and E antibodies.
IgG is the most common type of antibody. It is the chief
Ig against microbes. It acts by coating the microbe to hasten
its removal by other immune system cells. It gives lifetime or
long-standing immunity against infectious diseases. It is
highly mobile, passing out of the blood stream and between
cells, going from organs to the skin where it neutralizes sur-
face bacteria and other invading microorganisms. This mobil-
ity allows the antibody to pass through the placenta of the
mother to her fetus, thus conferring a temporary defense to the
unborn child.
After birth, IgG is passed along to the child through the
mother’s milk, assuming that she nurses the baby. But some of
the Ig will still be retained in the baby from the placental trans-
mission until it has time to develop its own antibodies.
Placental transfer of antibodies does not occur in horses, pigs,
cows, and sheep. They pass their antibodies to their offspring
only through their milk.
This antibody is found in body fluids such as tears,
saliva, and other bodily secretions. It is an antibody that pro-
vides a first line of defense against invading pathogens and
allergens, and is the body’s major defense against viruses. It is
found in large quantities in the bloodstream and protects other
wet surfaces of the body. While they have basic similarities,
each IgA is further differentiated to deal with the specific
types of invaders that are present at different openings of the
body.
Since this is the largest of the antibodies, it is effective
against larger microorganisms. Because of its large size (it
combines five Y-shaped units), it remains in the bloodstream
where it provides an early and diffuse protection against
invading antigens, while the more specific and effective IgG
antibodies are being produced by the plasma cells.
The ratio of IgM and IgG cells can indicate the various
stages of a disease. In an early stage of a disease there are
more IgM antibodies. The presence of a greater number of IgG
antibodies would indicate a later stage of the disease. IgM
antibodies usually form clusters that are in the shape of a star.
This antibody appears to act in conjunction with B and
T-cells to help them in location of antigens. Research contin-
ues on establishing more precise functions of this antibody.
The antibody responsible for allergic reactions, IgE acts
by attaching to cells in the skin called mast cells and basophil
cells (mast cells that circulate in the body). In the presence of
environmental antigens like pollens, foods, chemicals, and
drugs, IgE releases histamines from the mast cells. The hista-
mines cause the nasal inflammation(swollen tissues, running
nose, sneezing) and the other discomforts of hay fever or other
types of allergic responses, such as hives, asthma, and in rare
cases, anaphylactic shock (a life-threatening condition
brought on by an allergy to a drug or insect bite). An explana-
tion for the role of IgE in allergy is that it was an antibody that
was useful to early man to prepare the immune system to fightparasites. This function is presently overextended in reacting
to environmental antigens.
The presence of antibodies can be detected whenever
antigens such as bacteria or red blood cells are found to agglu-
tinate (clump together), or where they precipitate out of solu-
tion, or where there has been a stimulation of the plasma
complement system. Antibodies are also used in laboratory
tests for blood typing when transfusions are needed and in a
number of different types of clinical tests, such as the
Wassermann test for syphilisand tests for typhoid feverand
infectious mononucleosis.
By definition, anything that makes the immune system
respond to produce antibodies is an antigen. Antigens are liv-
ing foreign bodies such as viruses, bacteria, and fungithat
cause disease and infection. Or they can be dust, chemicals,
pollen grains, or food proteins that cause allergic reactions.
Antigens that cause allergic reactions are called aller-
gens. A large percentage of any population, in varying
degrees, is allergic to animals, fabrics, drugs, foods, and prod-
ucts for the home and industry. Not all antigens are foreign
bodies. They may be produced in the body itself. For example,
cancer cells are antigens that the body produces. In an attempt
to differentiate its “self” from foreign substances, the immune
system will reject an organ transplant that is trying to maintain
the body or a blood transfusion that is not of the same blood
type as itself.
There are some substances such as nylon, plastic, or
Teflon that rarely display antigenic properties. For that reason,
nonantigenic substances are used for artificial blood vessels,
component parts in heart pacemakers, and needles for hypo-
dermic syringes. These substances seldom trigger an immune
system response, but there are other substances that are highly
antigenic and will almost certainly cause an immune system
reaction. Practically everyone reacts to certain chemicals, for
example, the resin from the poison ivy plant, the venoms from
insect and reptile bites, solvents, formalin, and asbestos. Viral
and bacterial infections also generally trigger an antibody
response from the immune system. For most people penicillin
is not antigenic, but for some there can be an immunological
response that ranges from severe skin rashes to death.
Another type of antigen is found in the tissue cells of
organ transplants. If, for example, a kidney is transplanted, the
surface cells of the kidney contain antigens that the new host
body will begin to reject. These are called human leukocyte
antigens(HLA), and there are four major types of HLA subdi-
vided into further groups. In order to avoid organ rejection, tis-
sue samples are taken to see how well the new organ tissues
match for HLA compatibility with the recipient’s body. Drugs
will also be used to suppress and control the production of
helper/suppressor T-cells and the amount of antibodies.
Red blood cells with the ABO antigens pose a problem
when the need for blood transfusions arises. Before a transfu-
sion, the blood is tested for type so that a compatible type is
used. Type A blood has one kind of antigen and type B
another. A person with type AB blood has both the A and B
antigen. Type O blood has no antigens. A person with type A
blood would require either type A or O for a successful trans-
fusion. Type B and AB would be rejected. Type B blood wouldwomi_A 5/6/03 1:07 PM Page 27