DEVELOPMENT OF HUMORAL IMMUNITY

DEVELOPMENT OF HUMORAL IMMUNITY

Humoral immunity is defined as the immunity mediated by antibodies, which are secreted by B lymphocytes. B lymphocytes secrete the antibodies into the blood and lymph. The blood and lymph are the body fluids (humours or humors in Latin). Since the B lymphocytes provide immunity through humors, this type of immunity is called humoral immunity or B cell immunity. Antibodies are the gamma globulins produced by B lymphocytes. These antibodies fight against the invading organisms. The humoral immunity is the major defense mechanism against the bacterial infection. As in the case of cell-mediated immunity, the macrophages and other antigen-presenting cells play an important role in the development of humoral immunity also.

ROLE OF ANTIGEN-PRESENTING CELLS

The ingestion of foreign organisms and digestion of their antigen by the antigen-presenting cells are already explained.

Presentation of Antigen

Antigen-presenting cells present the antigenic products bound with HLA (which is present in class II MHC

molecule) to B cells. This activates the B cells through series of events.

Sequence of Events during Activation of B Cells

1. B cell recognizes the antigen displayed on the surface of the antigen-presenting cell, with the help

of its own surface receptor protein called B cell receptor.

2. Recognition of the antigen by the B cell initiates a complex interaction between the B cell receptor and

the antigen. This reaction activates B cells.

3. At the same time, macrophages (the antigen-presenting cells) release interleukin-1, which facilitates the activation and proliferation of B cells.

4. Activated B cells proliferate and the proliferated cells carry out the further actions.

5. Simultaneously, the antigen bound to class II MHC molecules activates the helper T cells, also resulting in development of cell-mediated immunity.

Transformation B Cells

Proliferated B cells are transformed into two types of cells:

1. Plasma cells

2. Memory cells.

ROLE OF PLASMA CELLS

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.

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.

ROLE OF HELPER T CELLS

Helper T cells are simultaneously activated by antigen. Activated helper T cells secrete two substances called interleukin-2 and B cell growth factor, which promote:

1. Activation of more number of B lymphocytes.

2. Proliferation of plasma cells.

3. Production of antibodies.

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