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T-CELL LEUKEMIA VIRUS•seeHUMANT-CELL
LEUKEMIA VIRUS(HTLV)T T cells or T-lymphocytesCELLS ORT-LYMPHOCYTES
When a vertebrate encounters substances that are capable of
causing it harm, a protective system known as the immune
systemcomes into play. This system is a network of many dif-
ferent organs that work together to recognize foreign sub-
stances and destroy them. The immune system can respond to
the presence of a disease-causing agent (pathogen) in two
ways. Immune cells called the B cellscan produce soluble pro-
teins (antibodies) that can accurately target and kill the
pathogen. This branch of immunityis called “humoral immu-
nity.” In cell-mediated immunity, immune cells known as the
T cells produce special chemicals that can specifically isolate
the pathogen and destroy it.
The T cells and the B cells together are called the lym-
phocytes. The precursors of both types of cells are produced in
the bone marrow. While the B cells mature in the bone mar-
row, the precursors to the T cells leave the bone marrow and
mature in the thymus. Hence the name, “T cells” for thymus-
derived cells.
The role of the T cells in the immune response is to
specifically recognize the pathogens that enter the body and to
destroy them. They do this either by directly killing the cells
that have been invaded by the pathogen, or by releasing solu-
ble chemicals called cytokines, which can stimulate other
killer cells specifically capable of destroying the pathogen.
During the process of maturation in the thymus, the T
cells are taught to discriminate between self (an individual’s
own body cells) and non-self (foreign cells or pathogens). The
immature T cells, while developing and differentiating in the
thymus, are exposed to the different thymic cells. Only those
T cells that are self-tolerant, that is to say, they will not inter-
act with the molecules normally expressed on the differentbody cells are allowed to leave the thymus. Cells that react
with the body’s own proteins are eliminated by a process
known as “clonal deletion.” The process of clonal deletion
ensures that the mature T cells, which circulate in the blood,
will not interact with or destroy an individual’s own tissues
and organs. The mature T cells can be divided into two sub-
sets, the T-4 cells (that have the accessory molecule CD4), or
the T-8 (that have CD8 as the accessory molecule).
There are millions of T cells in the body. Each T cell has
a unique protein structure on its surface known as the T cell
receptor (TCR), which is made before the cells ever encounter
an antigen. The TCR can recognize and bind only to a mole-
cule that has a complementary structure. It is kind of like a
lock-and key arrangement. Each TCR has a unique binding
site that can attach to a specific portion of the antigen called
the epitope. As stated before, the binding depends on the com-
plementarity of the surface of the receptor and the surface of
the epitope. If the binding surfaces are complementary, and the
T cells can effectively bind to the antigen, then it can set into
motion the immunological cascade which eventually results in
the destruction of the pathogen.
The first step in the destruction of the pathogen is the
activation of the T cells. Once the T-lymphocytes are acti-
vated, they are stimulated to multiply. Special cytokines called
interleukins that are produced by the T-4 lymphocytes mediate
this proliferation. It results in the production of thousands of
identical cells, all of which are specific for the original anti-
gen. This process of clonal proliferation ensures that enough
cells are produced to mount a successful immune response.
The large clone of identical lymphocytes then differentiates
into different cells that can destroy the original antigen.
The T-8 lymphocytes differentiate into cytotoxic T-lym-
phocytes (CTLs) that can destroy the body cells that have the
original antigenic epitope on its surface, e.g., bacterial
infected cells, viral infected cells, and tumor cells. Some of the
T lymphocytes become memory cells. These cells are capable
of remembering the original antigen. If the individual is
exposed to the same bacteriaor virus again, these memorywomi_T 5/7/03 11:02 AM Page 539