What are the primary classes of immunoglobulins?

There are five primary classes of immunoglobulins based on the type of heavy chain present in the molecule, which also gives each immunoglobulin a different function in immune response.  The five primary classes include IgG, IgM, IgA, IgD, and IgE. 

IgG - Immunoglobulin G 

IgG is the main antibody in the blood, making up about 80% of the total antibody content in the serum. It plays a key role in the biological defense system because of its powerful ability to bind toxins and bacteria. It is capable of recognizing the antibody-antigen complex and effectively neutralizing pathogenic substances. IgG is capable of passing through the placenta to the fetus, providing immunity to the developing fetus.  

IgM - Immunoglobulin M

IgM, which circulates in the blood and lymph, is the first antibody produced in response to pathogen invasion by B cells. It facilitates efficient activation of the immune system using the large number of antigenic sites on its surface and is also involved in agglutination and opsonization. Although IgM constitutes only 6% of the total antibody content in the serum, it plays an integral role in the initial immune system defense. 

IgA – Immunoglobulin A

IgA makes up 13% of the total antibody content in serum. It is usually found in bodily fluids such as serum, breast milk, saliva, nasal discharge, and intestinal fluids. IgA in breast milk is significant for its ability to provide infants’ gastrointestinal tracts with crucial protection against microbial activity. 

IgD – Immunoglobulin D

IgD is present on the surface of B cells and is involved in the production of antibodies by B cells. It also acts as a receptor on B cell surface, participates in activation and differentiation of B cells, and plays a key role in preventing respiratory tract infections. IgD makes up less than 1% of the total antibody content in serum.

IgE – Immunoglobulin E

IgE is present in the respiratory and intestinal tract linings and makes up only about 0.02% of the antibody content in the serum. IgE is thought to be involved in allergies such as pollinosis, because of its ability to bind to mast cells.