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ANTIBODIES

WHAT ARE ANTIBODIES ?

Antibodies or immunoglobulins are γ globulins which are produced in response to antigenic stimulation. These react specifically with the antigens which stimulated their production. All antibodies are Igs, but all Igs are not not antibodies.


HOW ARE ANtiBODIES PRODUCED ?

Antibodies are mainly produced by Plasma cells (upto 2000 molecules per second) and to some extent by B-lymphocytes also. These antibodies can be either bound to membrane or can be free.


The first antibody produced following an infection ( primary response ) are IgM , becoming detectable about 1 week after infection and persisting for about 6 weeks. About 2 weeks after infection , IgG antibody is produced and is long lasting. IgG is the main antibody formed in the secondary response.


STRUCTURE OF ANTIBODIES

The basic structural unit of all immunoglobulins contains a Y-shaped four polypeptide chain molecule. An antibody molecule consists of the following parts:-

  1. Heavy chains Two heavy chains of 50000 daltons molecular weight.

  2. Light chains Two light chains of 25000 daltons molecular weight.

  3. Disulphide bonds Light and Heavy chains are joined by a single disulphide bond while both Heavy chains are linked by two disulphide bonds.

  4. Constant and Variable regions Each chain of the antibody consist of a constant and a variable region.

  5. Fragment Antigen Binding (Fab) and Fragment Crystallizable (Fc) The Fab binds to specific antigens in a lock and key pattern forming an Antigen-antibody complex. The Fc part do not bind to antigen and can be crystallized.


TYPES OF ANTIBODIES

Antibodies can be divided into 5 classes based on their antigenic structure and physio-chemical nature i.e. IgG , IgA , IgD, IgE, IgM.


IMMUNOGLOBULIN G
  • Most abundant antibody ( approximately 75 %)

  • Occurs as a monomer

  • Found in blood, lymph and intestines

  • Major form of antibody produced during the secondary response

  • It is not produced by the foetus , but it can cross placental barrier. Hence, it is responsible for the protection of infant during first few months of life.

  • It usually exhibits high affinity for antigens leading to efficient neutralization of toxins.

  • IgG suppresses the homologous antibody synthesis by a feedback process. This property is utilised for prevention of isoimmunization of Rh -negative mother bearing Rh positive baby by administration of anti-Rh ( D) IgG at the time of delivery.



IMMUNOGLOBULIN A
  • It is the second most abundant class , approximately 15% of human serum immunoglobulins.

  • It exists in two forms - Monomeric (in serum) and Dimeric (secretory Ig A).

  • It is predominantly found in secretions such as- Milk, tears, nasal secretions, saliva, perspiration and serous secretions.

  • It plays an important role in local immunity against respiratory pathogens and intestinal pathogens.

  • Secretory Ig A when present in secretions prevent attachment of organisms to epithelial cells thus preventing adhesion, colonization and infection.

  • Secretory IgA is relatively resistant to digestive enzymes.


IMMUNOGLOBULIN M
  • Forms 5-10% of the total antibodies in the blood.

  • Occurs as a pentamer (at least 5 times larger than Ig G).

  • It is present on the surface of virtually all uncommitted B cells and remains exclusively in the serum and not found exvascularly in body.

  • Ig M is responsible for protection against blood invasion by microorganisms.

  • It is the main immunoglobulin in primary response.

  • It is a marker of acute infection.

  • It does not cross placenta but the foetus synthesises IgM by about 20 weeks of gestation.

  • IgM is much more efficient than Ig G in its ability to fix complement , promoting lysis and death of most gram negative bacteria.



IMMUNOGLOBULIN D
  • About 0.2% of the total antibodies in the blood.

  • Occurs as a monomeric immunoglobulin.

  • It is present on the surface of B lymphocytes which are destined to differentiate into antibody producing plasma cells.

  • It does not cross placenta.



IMMUNOGLOBULIN E
  • Less than 0.1% of the all antibodies in the blood

  • It is also a monomeric immunoglobulin.

  • IgE is mixed to the surface of mast cells and basophils and when a specific antigen binds to the Ig E , release of histamine and serotonin takes place.

  • It is chiefly produced in the linings of the respiratory and intestinal tracts.

  • It is heat labile and gets inactivated by heating at 56 degree celsius for 30 minutes.

  • IgE are involved in allergic and hypersensitivity reactions and also provides protection against parasitic worms

  • It does not cross placenta




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