Characteristics and Functions Antibodies

Antibodies show the following characteristics and perform different functions.

(i) IgA:

It is the second most abundant class, constituting about 10 to 15 per cent 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 secretory component. Levels decrease during stress, lowering resistance to infection. Provides localized protection in external secretions (tears, intestinal secretions, etc.) against bacteria and viruses. When IgA is excreted through faeces, it is called coproantibody.

(ii) IgD:

It is mainly found on the surfaces of В cells as antigen receptors, where it activates В cells for antigen recognition. It is about 0.2% of all antibodies in the blood.

(iii) IgE:

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 reactions; provides protection against parasitic worms. This immunoglobulin was discovered in 1966 by Ishizaka.

It exhibits unique properties such as heat lability (inactivated at 56°C in one hour). IgE mediates type I hypersensitivity (anaphylaxis). Prausnitz and Kustner in 1921 demonstrated transmission of IgE-mediated type I hypersensitivity. It is called Prausnitz-Kustner (PK) reaction. Thus IgE acts as mediator in allergic response.

(iv) IgG:

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.

(v) IgM:

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 В cells. It is also the earliest immunoglobin to be synthesised by the foetus, IgM has a J chain and its each dimer contains polypeptide called a secretory component.

It cannot cross the placental barrier. IgM is 500-1000 times more effective than IgG in opsonisation (to be described ahead), in bacterial action and in bacterial agglutination. But in neutralization of toxins and viruses, it is less active than IgG. It helps in complement activation.