Antibody:- Structure, Types and Function

Antibody:- Structure, Types and Function

Antibody (Ab) also know as Immunoglobulin (Ig) is the large Y shaped protein produced by the body's immune system when it detects harmful substances, called antigens like bacteria and viruses. The production of antibodies is a major function of the immune system and is carried out by a type of white blood cell called a B cell (B lymphocyte), differentiated B cells called plasma cells. The produced antibodies bind to specific antigensexpress in external factor and cancer cells.

Structure of Antibody:-
  Antibodies are heavy (~ 150 kDa) globular plasma proteins. The basic structure of all antibody are same.



          There are four polypeptide chains:-
▪Two identical heavy chains 
▪Two identical light chains
▪ Connected by Disulfide bonds.
        Light Chain (L) consists polypeptides of about 22,000 Da and Heavy  Chain (H) consists larger polypeptides of around 50,000 Da or more.
            There are five types of Ig heavy chain (in mammals). There are two types of Ig light chain (in mammals).

An antibody is made up of a variable region and a constant region and the region that changes to various structures depending on differences in antigens is called the variable region, and the region that has a constant structure is called the constant region.
     Each heavy and light chain in an immunoglobulin molecules contains an amino-terminal variable (V) region that consists of 100 to 110 amino acids and differ from one antibody to another.

Types Of Antibodies:-

1. IgA (Ig alpha):-  It is the second most abundant class, constituting about 10 to 15 percent of antibodies of serum. It is mainly found in sweat, tears, saliva,mucus, colostrum (first milk secreted by a mother) and gastrointestinal secretions.
  Smaller quantities are present in blood and lymph. IgA has an extra polypeptide called a J-(joining) chain and extra protein known as secondary component. Levels decrease during stress, lowering resistance to infection.

2. IgD (Ig delta):- It is mainly found on the surface og B cells as antigen receptors, where it activates B cells for antigen recongnition. It is about 0.2% of all antibodies in the blood.

3. IgE (Ig epsilon):-  It is less than 0.1% of all antibodies in the blood; located on mast cells and basophils releasing histamine from mast cells and basophils. It is involved in allergic and hypersensitivity reaction; provides protection against parasitic worms. This immunoglobulin was discovered in 1966 by Ishizaka.

4. IgG (Ig gamma):- This is the most abundant class of Ig in the body constituting approximately 80% of the total Igs. It is found in the blood, lymph and intestine. It protects against bacteria and viruses by enhancing phagocytosis, neutralizing toxins and complement activation. It is the only class of antibody to cross the placenta from mother to foetus thereby conferring considerable immune protection in new-borns.

5. IgM (Ig mu):- IgM is about 5 to 10% of all antibodies in the blood. It is also found in lymph. It is the largest Ig which is secreted first by the plasma cells. It is so named because it is a macroglobulin at least five times larger than IgG. IgM is the oldest immunoglobulin class. It activates the B cells. It is also the earliest immunoglobulin to be synthesised by the foetus, IgM a J chain and its each dimer contains polypeptide called a secretory component.

Functions of Antibody:-

■ IgG protect against bacteria, viruses, neutralise bacterial toxins, trigger compliment protein systems and bind antigen to enhance the effectiveness of phagocytosis.

■ IgG provides long-time protection because it persists for month and years after the presence of the antigen that has triggered their production.

■ Main function of IgA is to bind antigen on microbes before they invade tissues. It aggregates the antigens and keeps them in the secretion so when the secretion is expelled, so is the antigen.

■ IgA are also first defense for mucosal surface such as the intestines, nose, and lungs.