WORLD OF MICROBIOLOGY AND IMMUNOLOGY Immunodeficiency293•
plant, there can be a massive cascade reaction of antibody pro-
duction. This will include all of the immune responses of
which the body is capable. Such a massive response can over-
load the system and it can cause death. Thus, tissue matching
in organ transplants is vitally important. Often, a large range
of immunosuppressor drugs are employed until the body inte-
grates a particular organ. In some cases, this may necessitate a
course of drugs for the rest of the individuals life.
Histocompatibility problems also exist with blood.
Fortunately, the proteins in blood are less specific and blood
transfusions are a lot easier to perform than organ transplants.
The blood-typing systems that are in use are indications of the
proteins that are present. If blood is mixed from the wrong
types, it can cause lethal clotting. The main blood types are A,
B, O, and AB. Group O individuals are universal donors, they
can give blood to anyone. Group AB are universal recipients
because they can accept blood from anyone. Type A blood has
A antigens on the blood cells and B antibodies in the plasma.
The combination of B antibodies and B antigens will cause
agglutination. There are also subsidiary blood proteins such as
the rhesus factor (rh) that can be positive (present) or negative
(absent). If only small amounts of blood are transfused, it is
not a problem due to the dilution factor.
Immunochemistry is the chemistry of the immune sys-
tem. Most of the chemicals involved in immune responses are
proteins. Some chemicals inactivate invading proteins, others
facilitate this response. The histocompatibility complex is a
series of surface proteins on organs and tissues that elicit an
immune response when placed in a genetically different indi-
vidual.See also Biochemistry; History of immunology; Immune
stimulation, as a vaccine; Immunity, active, passive and
delayed; Immunity, cell mediated; Immunity, humoral regula-
tion; Immunization; Immunological analysis techniques;
Laboratory techniques in immunology; Major histocompati-
bility complex (MHC)IImmunodeficiencyMMUNODEFICIENCY
The immune systemis the body’s main system to fight infec-
tions. Any defect in the immune system decreases a person’s
ability to fight infections. A person with an immunodeficiency
disorder may get more frequent infections, heal more slowly,
and have a higher incidence of some cancers.
The normal immune system involves a complex inter-
action of certain types of cells that can recognize and attack
“foreign” invaders, such as bacteria, viruses, and fungi. It also
plays a role in fighting cancer. The immune system has both
innate and adaptive components. Innate immunityis made up
of immune protections present at birth. Adaptive immunity
develops the immune system to fight off specific invading
organisms throughout life. Adaptive immunity is divided into
two components: humoral immunity and cellular immunity.
The innate immune system is made up of the skin
(which acts as a barrier to prevent organisms from entering the
body), white blood cells called phagocytes, a system of pro-teins called the complementsystem, and chemicals called
interferons. When phagocytes encounter an invading organ-
ism, they surround and engulf it to destroy it. The complement
system also attacks bacteria. The elements in the complement
system create a hole in the outer layer of the target cell, which
leads to the death of the cell.
The adaptive component of the immune system is
extremely complex, and is still not entirely understood.
Basically, it has the ability to recognize an organism or tumor
cell as not being a normal part of the body, and to develop a
response to attempt to eliminate it.
The humoral response of adaptive immunity involves a
type of cell called B lymphocytes. B lymphocytes manufacture
proteins called antibodies (which are also called immunoglob-
ulins). Antibodies attach themselves to the invading foreign
substance. This allows the phagocytes to begin engulfing and
destroying the organism. The action of antibodies also acti-
vates the complement system. The humoral response is partic-
ularly useful for attacking bacteria.
The cellular response of adaptive immunity is useful for
attacking viruses, some parasites, and possibly cancer cells.
The main type of cell in the cellular response is T lympho-
cytes. There are helper T lymphocytes and killer T lympho-
cytes. The helper T lymphocytes play a role in recognizing
invading organisms, and they also help killer T lymphocytes to
multiply. As the name suggests, killer T lymphocytes act to
destroy the target organism.
Defects can occur in any component of the immune sys-
tem or in more than one component (combined immunodefi-
ciency). Different immunodeficiency diseases involve
different components of the immune system. The defects can
be inherited and/or present at birth (congenital), or acquired.
Congenital immunodeficiency is present at the time of
birth, and is the result of genetic defects. Even though more
than 70 different types of congenital immunodeficiency disor-
ders have been identified, they rarely occur. Congenital
immunodeficiencies may occur as a result of defects in B lym-
phocytes, T lymphocytes, or both. They can also occur in the
innate immune system.
If there is an abnormality in either the development or
function of B lymphocytes, the ability to make antibodies will
be impaired. This allows the body to be susceptible to recur-
rent infections. Bruton’s agammaglobulinemia, also known as
X-linked agammaglobulinemia, is one of the most common
congenital immunodeficiency disorders. The defect results in
a decrease or absence of B lymphocytes, and therefore a
decreased ability to make antibodies. People with this disorder
are particularly susceptible to infections of the throat, skin,
middle ear, and lungs. It is seen only in males because it is
caused by a genetic defect on the X chromosome. Since males
have only one X chromosome, they always have the defect if
the geneis present. Females can have the defective gene, but
since they have two X chromosomes, there will be a normal
gene on the other X chromosome to counter it. Women may
pass the defective gene on to their male children.
Another type of B lymphocyte deficiency involves a
group of disorders called selective immunoglobulin deficiency
syndromes. Immunoglobulin is another name for antibody,womi_I 5/6/03 3:23 PM Page 293