Immunity is defined as the capacity of the body to resist pathogenic agents. It is the ability of body to resist the entry of different types of foreign bodies like bacteria, virus, toxic substances, etc

Immunity is of two types-

  1. Innate immunity.
  2. Acquired immunity.

Innate immunity-

Innate immunity is the inborn capacity of the body to resist pathogens. By chance, if the organisms enter the body, innate immunity eliminates them before the development of any disease. It is otherwise called the Natural or Non-specific immunity.

This type of immunity represents the first line of defence against any type of pathogens. Therefore, it is also called non-specific immunity.

Acquired immunity-

Acquired immunity is the resistance developed in the body against any specific foreign body like bacteria, viruses, toxins, vaccines or transplanted tissues. So, this type of immunity is also known as specific immunity.

It is the most powerful immune mechanism that protects the body from the invading organisms or toxic substances. Lymphocytes are responsible for acquired immunity.

Types of Acquired Immunity Two types of acquired immunity develop in the body-

  1. Cellular immunity
  2. Humoral immunity.

Lymphocytes are responsible for the development of these two types of immunity.

The two categories are-

  • T lymphocytes or T cells, which are responsible for the development of cellular immunity
  • B lymphocytes or B cells, which are responsible for humoral immunity.

T lymphocytes-

Thymus secretes a hormone called thymosin, which plays an important role in immunity. It accelerates the proliferation and activation of lymphocytes in thymus. It also increases the activity of lymphocytes in lymphoid tissues.

T lymphocytes are transformed into four types-

  1. Helper T cells or inducer T cells. These cells are also called CD4 cells because of the presence of molecules called CD4 on their surface.

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:

  • Activation of B cells.
  • Proliferation of plasma cells.
  • Production of antibodies by plasma cell.

2. Cytotoxic T cells or killer T cells. These cells are also called CD8 cells because of the presence of molecules called CD8 on their surface.

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 -

  • Receptors situated on the outer membrane of cytotoxic T cells bind the antigens or organisms tightly with cytotoxic T cells.
  • Then, the cytotoxic T cells enlarge and release cytotoxic substances like the lysosomal enzymes.
  • These substances destroy the invading organisms.
  • Like this, each cytotoxic T cell can destroy a large number of microorganisms one after another.

Other Actions of Cytotoxic T Cells

  • 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.
  • 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.

3. Suppressor T cells.

Role of Suppressor T cell-

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.

4. Memory T cells.

Role of memory T cells-

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.

Storage of T Lymphocytes-

After the transformation, all the types of T lymphocytes leave the thymus and are stored in lymphoid tissues of lymph nodes, spleen, bone marrow and GI tract.

B lymphocytes-

 B lymphocytes were first discovered in the bursa of Fabricius in birds, hence the name B lymphocytes. Bursa of Fabricius is a lymphoid organ situated near the cloaca of birds. Bursa is absent in mammals and the processing of B lymphocytes takes place in liver (during fetal life) and bone marrow (after birth).

Types of B Lymphocytes After processing, the B lymphocytes are transformed into two types:

  1. Plasma cells.

Role of plasma cell-

Plasma cells destroy the foreign organisms by producing the antibodies. Antibodies are globulin in nature. The rate of the antibody production is very high, i.e. each plasma cell produces about 2000 molecules of antibodies per second. The antibodies are also called immunoglobulins. Antibodies are released into lymph and then transported into the circulation. The antibodies are produced until the end of lifespan of each plasma cell, which may be from several days to several weeks.

2. Memory cells.

Role of Memory B cells-

 Memory B cells occupy the lymphoid tissues throughout the body. The memory cells are in inactive condition until the body is exposed to the same organism for the second time. During the second exposure, the memory cells are stimulated by the antigen and produce more quantity of antibodies at a faster rate, than in the first exposure. The antibodies produced during the second exposure to the foreign antigen are also more potent than those produced during first exposure. This phenomenon forms the basic principle of vaccination against the infections

Storage of B Lymphocytes-

 After transformation, the B lymphocytes are stored in the lymphoid tissues of lymph nodes, spleen, bone marrow and the GI tract.