Definition of Antibodies-properties,structure and classification of Antibodies (IgG,IgA,IgM,IgD,IgE)

ANTIBODIES :

 Antibodies are substances which are formed in the serum and tissue fluids in response to an antigen and react with that antigen specifically and in some observable manner.

A.    Properties of antibodies :

Chemical nature of antibodies is globulin and they are named as immunoglobulins . immunoglobulins are mainly synthesized by plasma cells.

B.     Structure of immunoglobulin :-

1.      An antibody molecule consists of two identical heavy and two identical chains.

2.      The heavy (H) chains are shorter. Both types of chains are polypeptide in nature.

3.      The two heavy chains are held together by disulphide (S-S) bonds.

4.      Each light chain is also attached to heavy chain by disulphide bond.

5.      The H chains are structurally and antigenically distinct in different classes namely IgG, IgM,IgA,IgD, and igE are designated depending on presence of heavy chain, gamma (¥) , mu(µ), alpha (α), delta (Ꝺ) and epsilon (€) respectively.

6.      The  L chains are  similar in all  classes of immunoglobulins.

7.      They are present in two forms kappa (K) and lambda (L).

8.      Each immunoglobulin has either two kappa or two lambda light chains but both (K&L) are never found together in a molecule.

9.      Both L and H chains consist of two portions each, a variable (V) region and a constant (C) region.

10.  Antigen combining site is at its amino terminus which consists of both H and L chains.

Classification :

 Immunoglobulin are classified into 5 types.

1.      Immunoglobulin G (IgG)

2.      Immunoglobulin A (IgA)

3.      Immunoglobulin M (IgM)

4.      Immunoglobulin D (IgD)

5.      Immunoglobulin E (IgE)

1.      Immunoglobulin G (IgG)

·         IgG is the major serum immunoglobulin . the  normal serum concentration is about 8-16 mg/dl.

·         Molecular weight is 150,000 (7S).

·         It is the only immunoglobulin that is transported through placenta and provides natural passive immunity to newborn.

·         It is distributed equally between the intravascular and extravascular compartments.

·         IgG appears late but persists for longer period. It appears after the initial immune response which is IgM in nature.

·         It participates in precipitation, complement fixation and neutralization of toxin and viruses.

·         It is protective against those microorganisms which are active in the blood and tissues.

2.      Immunoglobulin A (IgA)

·    

     IgA is the second major serum immunoglobulin (about 10-13% of serum immunoglobulins). The normal serum concentration is 0.6- 4.2 mg/ml.

·         IgA occurs in two forms, serum IgA and secretory IgA.

·         Serum IgA is a monomeric 7S molecule , while IgA found on mucosal surfaces and in secretions ( secretory IgA, MW 400,000) is a dimer formed by two monomer units joined  together by a glycoprotein named the J chain (J for joining).

·         Secretory IgA contains another polypeptide called the secretory piece or secretory component.

·         IgA is the principal immunoglobulin present in secretions such as milk, saliva, tears, sweat, nasal fluids, colostrum and in secretions of respiratory intestinal and genital systems. It protects the mucous membranes against microorganisms.

·         IgA is mainly  synthesized locally by plasma cells and little is derived from serum.

3.      Immunoglobulin M (IgM)

·          IgM is a pentamer consisting of 5 immunoglobulin subunits and one molecule of J chain,  which joins the Fc region of the basic subunits.

·         It constitutes about 5-8 percent of total serum immunoglobulins. The normal level in serum is 0.5 – 2 mg/ml.

·         It is heavy molecule (19S) with a molecular weight 900,000 to 1,000,000 hence also called the ‘ millionaire molecule’.

·         IgM is mainly distributed intravascularly (80%).

·         It is the earliest synthesized immunoglobulin by foetus.

·         It appears early in response to infection before IgG.  IgM antibodies are short lived,and disappear earlier than IgG. Hence , its presence in serum indicates recent infection.

·         It cannot cross the placenta , presence of IgM antibody in serum indicates congenital infection.

·         It is very effective antibody in agglutination and complement fixation. It is more efficient than IgG in these reactions.

·         IgM provides protection against blood invasion by microorganisms.

4.      immunoglobulin D (IgD)

·         IgD resembles IgG structurally .

·         IgD is present in a concentration of 3 mg per 100 ml in serum. It is mostly intravascular in distribution.

·         IgD (175kD) constitutes less than 1% (40mg/ml) of the antibody in human serum.

·         IgD is an antibody whose function remains unknown, even though it is one of the main receptors on mature B cells.

·         As B cells mature, IgD is replaced by other antibodies.

·         igD may be a regulator of immune responses.

·         The hinge region of IgD consists of 64 amino acid residues, longer than any other antibody class.

5.      Immunoglobulin E (IgE):-

·         Human IgE (190 kD) makes up less than 0.003% of the antibody in serum.

·         IgE binds through its Fc part to mast cells or basophils.

·         On later exposure to the same antigen, that antigen may be bound on mast cells and basophils with membrane – bound on mast cells and basophils with membrane – bound IgE and trigger allergic reactions.

·         As it mediates the immediate hyper sensitivity .

·         Reaction it is called regain antibody

·         IgE protects against parasites by releasing mediators that attract eosinophil.

·         Like the m chain, the 5 chain contains four C-region domains.

·         igE is made up of about 13% of carbohydrate.