Question

list and describe the role and use of antibody isotypes in the human immune system. Include functions, roles, locations.

Answer 1

Ans.

Antibodies are immunoglobulin (Ig) molecules that are made up of two large heavy chains and two small light chains,where the heavy chains are bound to the light chains to form a Y shaped structure by sulfhydryl linkages.There are two regions,a constant region (Fc) on the stem of the Y and a variable region (Fab) on the two prongs of the Y.Fab which interacts with the antigen is unique to each antibody,whereas Fc is common to all the antibodies and interact with the immune system.The class of the antibodies are defined by the Fc portion of the heavy chains and there are 5 such classes called as the isotypes of antibodies.They are:

1. Immunoglobulin G or IgG - It is the most common type of antibody that is found in blood circulation and constitutes around 75% of serum antibodies in humans and 80% of the total antibodies.It provides the maximum of the antibody based immunity against pathogens such as viruses,fungi,bacteria etc.
2. Immunoglobulin A or IgA - It is an important antibody that plays a vital role in the immune functions of mucous membranes.It is present in the respiratory mucosal secretions,urinary and gastrointestinal tracts,tears,saliva,milk and in serum.It functions by neutralizing the bacterial toxins and inhibiting its adhesion to the epithelial cells,thus protecting the mucosal surfaces.
3. Immunoglobulin M or IgM - It is the largest antibody that appears in first line with the initial exposure to antigens.It has got five Y structures that is joined by their Fc regions in a circular manner.It is present in the serum and is expressed on the surface of B cells.It makes up around 10% of the antibodies in the blood.It interacts with several physiological molecules and helps in various functions like opsonization of antigens and cytosis.
4. Immunoglobulin D or IgD -It is found on the B-lymphocytes surface.Together with IgM,it act as an antigen receptor for the B-cells activation.It makes up around 1% of the proteins present in the plasma membranes of immature B-Cells.
5. Immunoglobulin E or IgE - Synthesized by the plasma cells, IgE account for just 0.002% of total serum antibodies.This is a monomeric antibody that is bound to the tissue cells,specially the mast cells and are associated with allergic reactions.It plays an important role in response to various allergens.