ROLE OF HELPER T CELLS

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.