Question

What advantages/disadvantages do making different classes of antibodies have?

Answer 1

The human body is constantly infected with different microorganism such as viruses, bacteria, parasitic worms, protozoa etc. The human body produces five different types of antibodies to protect against these infections. All these antibodies have varying heavy chains. Different heavy chains are as follows:

IgG: γ-chains, IgMs: μ-chains, IgAs: α-chains, IgEs: ε-chains, IgDs: δ-chains

These variations are involved in functional responses of the antibodies and are determined by the Fc region of the antibody. IgA is present in mucosal membranes of respiratory tract .It has two sub classes, IgA1 and IgA2. IgD is present on the surface of B cells and in low amounts in blood. IgE is present in membranes and skin. It is produced during parasitic infections and allergic responses. IgG is the main antibody produced during bacterial infection and is around 75% of the total Igs produced. This antibody can cross the placental barrier. IgM is required for blood infection and can trigger IgG response. It is the first antibody made by the fetus.

Antibodies are produced by humans even when there is no immune attack. They are important in immune defense as they can cross react with different antigenic determinants. When the B cells are stimulated with repeated attacks of the antigen, the response is enhanced. IgM is the first antibody made during primary response. It can bind complement proteins effectively and has an important role to prevent bacterial infections. As the response proceeds, there is production of IgG, IgA, or IgE due to class switching. IgG is the main antibody of secondary responses. IgG is a monomeric antibody and can eliminate pathogens from the blood. It can and can activate complement proteins, thereby preventing bacterial infections, though not as effectively as IgM. It is important antibody in immune defense in fetus as it crosses the placental barrier. IgA protects the body surfaces from immune attack. This antibody is present in breast milk and protects the baby from humans. As IgE is required for allergic responses, it is present in low levels in blood. The IgE Fc portion can bind specifically to the IgE specific Fc receptor that is present on surfaces of mast cells. If a person is allergic to some chemical or food, production of IgE can be disadvantages and may even result in death of the individual. IgM and IgD are the only two antibodies that can bind naïve B cells. Functions of IgD are still unclear.

Viral infections generate IgG, IgM, and IgA, which neutralize the virus by blocking host-virus interaction and cause antibody-dependent cytotoxic cells (ADCC) or complement-mediated lysis due to recognition of antigens on virus surface.

Excessive production of antibodies, especially in absence of active infection can also damage the human body. Similarly, decreased production can have an effect too. Patients with congenital deficiencies have low levels of IgD. Monoclonal increase in production can cause formation of myelomas. Autoimmune Diseases such as systemic lupus erythematosus and rheumatoid arthritis are due to excess production of IgM and IgG antibodies. These antibodies will cause damage to tissues, as they react against self-antigens, which normally doesn’t happen. Low levels of IgA causes kidney damage, leukemia or defects in the intestine. Low levels of IgE is important in development of ataxia-telangiectasia that affects muscle coordination. High levels of IgG are high in myeloma, multiple sclerosis.