Antibodies-types,sturucture and their function.

Antibody simple definition- Humoral immunity begins as antigenic determinants from macrophages stimulate B- lymphocytes. The B-lymphocytes do not enter the circulation. Rather, they remain in the lymphoid tissue and multiple to form a clone of cells called plasma cells. Plasma cells are about two to three times the size of B- lymphocytes and have elaborate internal details. Their principal products are protein molecules called antibodies. Antibodies are synthesized according to directions in the IR genes and are released into the circulation at a rate of several thousand molecules per second.

Antibodies in blood

Plasma cells continue to produce antibodies for two to three days or until the antigenic stimulation comes to an end. Antibodies are produced by plasma cells.

Immunoglobulin 

Antibody molecules represent about 17 % of the total protein in the bloodstream. The enormous variety of antibodies should be evident from only a single fact that a single bacterium may have a thousand different antigens. And that each antigen might elicit its own highly specific antibody. The term immunoglobulin (Ig) is used synonymously with antibody because antibody molecules exhibit the characteristics of the globulin group of protein. 

All antibodies are proteins and are referred to as immunoglobulins. Five major classes of immunoglobulins can be distinguished based on their character­istic structures and separate functional roles.

Types of antibodies

1.  immunoglobulins (IgG),
2.  immunoglobulins (IgM),
3.  immunoglobulins (IgA),
4.  immunoglobulins (IgD),
5. and immunoglobulins (IgE).
6. 
   

IgG immunoglobulins constitute about 80% of all the antibodies in the blood serum. These are the major anti­bodies involved in the destruction of invading micro­organisms.

IgM immunoglobulins appear first in response to an antigen; they are later replaced by IgG.

IgA immunoglobulins are found in secretions such as saliva and tears and are involved in the defense of body cavities such as the mouth and vagina.

The function of IgD is still unknown,

while IgE is thought to be involved in allergic reactions.

Immunoglobulin structure

There are thus five basic types of immunoglobulins. Antibodies are immune system-related proteins called immunoglobulins. Each antibody consists of four polypeptides- two heavy chains and two light chains joined to form a 'Y ' shaped molecule. The basic antibody molecule consists of four polypeptide chains (two identical heavy H chains and two identical light L chains) these chains are joined together by sulfur (disulfide)linkages to form a Y -shaped structure. Each heavy chain consists of about 200 amino acids. Within each polypeptide chain there exist constant and variable regions. The amino acid in the constant region of both light and heavy chains are virtually identical among antibodies. However, the amino acids of the variable region vary among the hundreds of thousands of different antibodies. Thus, the variable region of light and heavy chain combine to form a highly specific, three-dimensional structure somewhat analogous to the active site of an enzyme. This portion of the antibody molecule combines with the antigen. The arms of the antibody are identical so that a single antibody molecule may combine with two antigen molecules. This combination may lead to a complex antibody and antigen molecules.

Antibody Neutralization and function 

The ends of each arm of the IgG molecule contain an active site that can combine with a specific antigen. Thus, two identical binding sites permit each antibody molecule to form a complex with two antigens. Both light and heavy chains contain two regions: a "constant" region that has very similar amino acid sequences in all immuno­globulins, and a "variable" region that has different amino acid sequences for each type of antibody. This variability accounts for the specificity of antibodies- that is, one particular antibody molecule can bind only one particular kind of antigen. The specific sequence of amino acids in the variable region at the end of each arm of the antibody molecule determines the spatial configuration of the active site which will only bind one particular antigen.


Certain antibodies called neutralizing antibodies, they react with antigens and prevent the antigens(bacteria, viruses) to attach them to host cells. Neutralizing antibodies also bind viruses together in clumps thereby encouraging phagocytosis. More ever neutralization represents a vital defense mechanism to toxins. The antibodies called antitoxins. After the toxin molecules near their active site neutralization of toxin molecules also increases their size.

Agglutinins 

some antibodies called agglutinins react with antigens on the surface of organisms .this action causes clumping or agglutination of the organisms and enhances phagocytosis.