ROLE OF HELPER T CELLS
Helper T cells (CD4
cells) which enter the circulation activate all the other T cells and B cells.
Normal, CD4 count in healthy adults varies between 500 and 1500 per cubic
millimeter of blood.
Helper T cells are of
two types:
1. Helper-1 (TH1)
cells
2. Helper-2 (TH2)
cells.
Role of TH1 Cells
TH1 cells are concerned
with cellular immunity and secrete two substances:
i. Interleukin-2,
which activates the other T cells.
ii. Gamma interferon, which
stimulates the phagocytic activity of cytotoxic cells, macrophages and
natural killer (NK)
cells.
Role of TH2 Cells
TH2 cells are concerned
with humoral immunity and secrete interleukin-4 and interleukin-5, which are
concerned with:
i. Activation of B
cells.
ii. Proliferation of
plasma cells.
iii. Production of
antibodies by plasma cell.
ROLE OF CYTOTOXIC T
CELLS
Cytotoxic T cells that
are activated by helper T cells, circulate through blood, lymph and lymphatic
tissues
and destroy the invading
organisms by attacking them directly.
Mechanism of Action
of Cytotoxic T Cells
1. Receptors situated
on the outer membrane of cytotoxic T cells bind the antigens or organisms
tightly
with cytotoxic T
cells.
2. Then, the cytotoxic
T cells enlarge and release cytotoxic substances like the lysosomal enzymes.
3. These substances
destroy the invading organisms.
4. Like this, each
cytotoxic T cell can destroy a large number of microorganisms one after
another.
Other Actions of
Cytotoxic T Cells
1. Cytotoxic T cells
also destroy cancer cells, transplanted cells, such as those of transplanted
heart or
kidney or any other
cells, which are foreign bodies.
2. Cytotoxic T cells
destroy even body’s own tissues which are affected by the foreign bodies,
particularly
the viruses. Many
viruses are entrapped in the membrane of affected cells. The antigen of the
viruses attracts the T
cells. And the cytotoxic T cells kill the affected cells also along with
viruses. Because of this, the cytotoxic T cell is called killer cell.
ROLE OF SUPPRESSOR T
CELLS
Suppressor T cells are
also called regulatory T cells. These T cells suppress the activities of the
killer T cells. Thus, the suppressor T cells play an important role in preventing
the killer T cells from destroying the body’s own tissues along with invaded
organisms. Suppressor cells suppress the activities of helper T cells also.
ROLE OF MEMORY T
CELLS
Some of the T cells
activated by an antigen do not enter the circulation but remain in lymphoid
tissue. These T cells are called memory T cells. In later periods, the memory
cells migrate to various lymphoid tissues throughout the body. When the body is
exposed to the same organism for the second time,
the memory cells
identify the organism and immediately activate the other T cells. So, the
invading organism is destroyed very quickly. The response of the T cells is also
more powerful this time.
SPECIFICITY OF T
CELLS
Each T cell is
designed to be activated only by one type of antigen. It is capable of
developing immunity against that antigen only. This property is called the
specificity of T cells.
2. Cytotoxic T cells
destroy even body’s own tissues which are affected by the foreign bodies,
particularly
the viruses. Many
viruses are entrapped in the membrane of affected cells. The antigen of the
viruses attracts the T
cells. And the cytotoxic T cells kill the affected cells also along with
viruses. Because of this, the cytotoxic T cell is called killer cell.
ROLE OF SUPPRESSOR T
CELLS
Suppressor T cells are
also called regulatory T cells. These T cells suppress the activities of the
killer T cells. Thus, the suppressor T cells play an important role in preventing
the killer T cells from destroying the body’s own tissues along with invaded
organisms. Suppressor cells suppress the activities of helper T cells also.
ROLE OF MEMORY T
CELLS
Some of the T cells
activated by an antigen do not enter the circulation but remain in lymphoid
tissue. These T cells are called memory T cells. In later periods, the memory
cells migrate to various
lymphoid tissues
throughout the body. When the body is exposed to the same organism for the
second time, the memory cells identify the organism and immediately activate
the other T cells. So, the invading organism is destroyed very quickly. The
response of the T cells is also more powerful this time.
SPECIFICITY OF T
CELLS
Each T cell is
designed to be activated only by one type of antigen. It is capable of
developing immunity against that antigen only. This property is called the
specificity of T cells.
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