IDF Patient & Family Handbook for Primary Immunodeficiency ... - IDFA

4 The Immune System And Primary Immunodeficiency Diseases
Components of the Immune System continued
Antibodies protect the host against infection in
a number of different ways. For example, some
microorganisms, such as viruses, must attach
to body cells before they can cause an infection,
but antibody bound to the surface of a virus can
interfere with the virus’s ability to attach to the host
cell. In addition, antibody attached to the surface
of some microorganisms can cause the activation
of a group of proteins, called the complement
system, that directly kills the bacteria or viruses.
Antibody-coated bacteria are also much easier
for phagocytic cells to ingest and kill than bacteria
that are not coated with antibody. All of these
actions of antibodies prevent microorganisms from
successfully invading body tissues and causing
serious infections. The long life of B-lymphocytes
enables us to retain immunity to viruses and
bacteria that infected us many years ago. For
example, once a person has been infected with
chicken pox, he or she will seldom catch it again
because they retain the B-lymphocytes and
antibodies for many years and the antibodies
prevent infection a second time.
T-lymphocytes
T-lymphocytes (sometimes called T-cells) are
another type of immune cell. T-lymphocytes do not
produce antibody molecules. The specialized roles
of T-lymphocytes are to directly attack foreign
antigens such as viruses, fungi, or transplanted
tissues, and to act as regulators of the immune
system. T-lymphocytes develop from stem cells in
the bone marrow. Early in fetal life, the immature
cells migrate to the thymus, a specialized organ
of the immune system in the chest. Within the
thymus, immature lymphocytes develop into
mature T-lymphocytes (the “T” stands for the
thymus). The thymus is essential for this process,
and T-lymphocytes cannot develop if the fetus
does not have a thymus. Mature T-lymphocytes
leave the thymus and populate other organs
of the immune system, such as the spleen,
lymph nodes, bone marrow, and blood. Each
T-lymphocyte reacts with a specific antigen, just
as each antibody molecule reacts with a specific
antigen. In fact, T-lymphocytes have molecules
on their surfaces that are similar to antibodies
and recognize antigens. The variety of different
T-lymphocytes is so extensive that the body has
T-lymphocytes that can react against virtually
any antigen.
T-lymphocytes also vary in their function. There
are “killer” or cytotoxic T-lymphocytes, helper
T-lymphocytes, and regulatory T-lymphocytes.
Each has a different role to play in the immune
system. Killer, or cytotoxic, T-lymphocytes are
the T-lymphocytes which perform the actual
destruction of the invading microorganism. Killer
T-lymphocytes protect the body from certain
bacteria and viruses that have the ability to survive
and even reproduce within the body’s own cells.
Killer T-lymphocytes also respond to foreign
tissues in the body, such as a transplanted kidney.
Killer T-lymphocytes migrate to the site of an
infection or the transplanted tissues. Once there,
the killer cell directly binds to its target and kills it.
Helper T-lymphocytes assist B-lymphocytes in
producing antibody and assist killer T-lymphocytes
in their attack on foreign substances. The helper
T-lymphocyte “helps” or enhances the function of
B-lymphocytes, causing them to produce more
antibodies more quickly and switch from the
production of IgM to IgG and IgA.
Regulatory T-lymphocytes suppress or turn off
other T-lymphocytes. Without regulatory cells, the
immune system would keep working even after
an infection had been cured and overreact to the
infection. Regulatory T-lymphocytes act as the
thermostat of the lymphocyte system to keep it
turned on just enough—not too much and not
too little.
NK Cells
NK cells are so named because they easily
kill cells infected with viruses. They are said
to be Natural Killer (NK) cells as they do not
require the same thymic education process that
T-lymphocytes require. NK cells are derived from
the bone marrow and are present in relatively
low numbers in the bloodstream and in tissues.
They are extremely important in defending against
viruses and many people believe that they act to
prevent cancer.
They act to kill viruses by attaching to the cell
that contains the virus and injecting it with a
killer potion of chemicals. They are particularly
important in the defense against herpes viruses.
This family of viruses includes the traditional cold
sore form of herpes as well as Epstein Barr virus
(the cause of most infectious mononucleosis) and
the varicella virus which causes chicken pox.