DEVELOPMENT OF HUMORAL IMMUNITY
INTRODUCTION
Humoral immunity is developed by the antibodies, which are circulating in the blood. The antibodies are the gamma globulins produced by В lymphocytes. These antibodies light against the invading organisms. The humoral immunity is the major defensive mechanism against the bacterial infection. As in the case of cellular immunity, the macrophages and other antigen presenting cells play an important role in the development of humoral immunity also.
ROLE OF ANTIGEN PRESENTING CELLS
When foreign bodies or organisms invade, macrophages and other antigen presenting cells destroy them mostly by phagocytosis. Then, the antigen from the organisms is digested into polypeptides. The polypeptide products are presented to В lymphocytes (and also to T lymphocytes) along with human leukocyte antigens (HLAs).
Now, the antigenic products activate the В lymphocytes and also the helper T cells. The macrophages also secrete some substance called interleukin 1. This causes activation and proliferation of lymphocytes.
ROLE OF PLASMA CELLS
The В lymphocytes are proliferated and transformed into two types of cells namely, plasma cells and memory cells. The plasma cells produce the antibodies, which are globulin in nature. The antibodies are called immunoglobulins. 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 released into lymph and then transported into the circulation. The antibodies are produced until the end of lifespan of each plasma cell, i.e. from several days to several weeks.
ROLE OF MEMORY В CELLS
Some of the В lymphocytes activated by the antigen are transformed into memory В cells, which 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 forms the basic principle of vaccination against the infections.
ROLE OF HELPER T CELLS
Helper T cells are simultaneously activated by the antigen. The activated helper T cells secrete two substances called interleukin 2 and В cell growth factor, which promote:
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1. Activation of more number of В lymphocytes
2. Proliferation of plasma cells and
3. Production of antibodies
ANTIBODIES
Antibodies or immunoglobulins (Ig) are produced by plasma cells in response to the presence of antigens. The immunoglobulins form 20% of the total plasma proteins. Though produced by В lymphocytes, the antibodies are found in almost all the tissues of the body.
Types of Antibodies
Five types of antibodies are known.
1. IgA (Ig alpha)
2. IgD (Ig delta)
3. IgE (Ig epsilon)
4. IgG (Ig gamma) and
5. IgM (Ig mu)
Among these antibodies, IgG forms 75% of the antibodies in the body.
Structure of Antibodies
Antibodies are gamma globulins with a molecular weight of 1, 50,000 to 9,00,000. The antibodies are formed by two pairs of chains namely, one pair of heavy or long chains and one pair of light or short chains. Each heavy chain consists of about 400 amino acids and each light chain consists of about 200 amino acids.
Actually, each antibody has two halves, which are identical. The two halves are held together by disulfide bonds (S-S). Each half of the antibody consists of one heavy chain (H) and one light chain (L). The two chains in each half are also joined by disulfide bonds (S-S). The disulfide bonds allow the movement of the amino acid chains. In each of the antibody, the light chain is parallel to one end of the heavy chain. The light chain and the part of heavy chain parallel to it form one arm. The remaining part of the heavy chain forms another arm. A hinge joins both the arms (Fig. 10).
Each chain of the antibody includes two regions.
1. Constant region and
2. Variable region
1. Constant Region
The amino acids present in this region are similar in number and placement (sequence) in all the antibodies of each type. So, this region is called constant region or Fc (Fragment crystalline) region. Thus, the identification and the functions of different types of immunoglobulins depend upon the constant region. This region binds to the antibody receptor situated on the surface of the cell membrane. It also causes complement fixation. So, this region is also called the complement binding region.
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2. Variable Region
It is smaller compared to constant region. The amino acids occupying this region are different in number and placement (sequence) in each antibody. So, it is called variable region. This region enables the antibody to recognize the specific antigen and to bind itself with the antigen. Because of this, this region of the chain is called antigen binding region or Fab (Fragment antigen binding) region.
Functions of Antibodies
The functions of the antibodies are:
1. IgA takes part in localized defense mechanism external secretions like tear
2. IgD is involved in recognition of the antigen and represents receptor of lymphocytes
3. IgE is involved in allergic reactions
4. IgG is responsible for complement fixation.
5. IgM is also responsible for complement fixation
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