Question

Describe the 5 classes of antibodies, including their function and a location for each.

Answer 1

Immunoglobulins commonly known as antibodies are Y shaped glycoprotein molecules produced by plasma cells which in turn are produced by B lymphocytes.

Each antibody Fab fragments and Fc fragments in their structure.

Fab is the antigen binding fragment which has a site called paratope which binds to a specific site on the antigen molecule called epitope. This helps the antibody to bind and tag the antigen which helps in their recognition and destruction by phagocytic cells of immune system.

Fc fragment is the constant region of the antibody which helps it in communication with other cells of immune system and also activates the complement cascade of immune system.

There are five classes of Immunoglobulins/antibodies depending upon their structure, function, site of action, etc.

i) IgG

It is the most abundant immunoglobulin distributed in the blood and interstitial fluids. It is the main immunoglobulin in the secondary immune response.

It is the only immunoglobulin which crosses the placenta hence important in neonatal protection. It also opsonises (tags) the antigens like bacteria for their recognition and killing by immune cells. It neutralizes bacterial toxins and viruses and also activates complement cascade of immune system.

ii) IgA

It is also known as secretory immunoglobulin as it is the principal immunoglobulin present in secretions of respiratory and gastrointestinal tract as well as in sweat, saliva, tears and colostrum.

It is important in mucosal defence mechanism as it prevents the attachment of viruses and bacteria to the mucosal surfaces. It also activates complement cascade of immune system.

iii) IgM

It is the largest and heaviest immunoglobulin made up of five monomeric units of immunoglobulins. It is a pentameric molecule.

It is mainly an intravascular immunoglobulin due to its large size. It is the main immunoglobulin in primary immune response.

It also activates the complement cascade of immune system. Blood group antibodies belong to IgM class of Immunoglobulins.

iv) IgD

It is found on the surface of many B cells and in serum.

It activates basophils and mast cells to produce antimicrobial proteins.

v) IgE

It is the only heat labile Immunoglobulin.

It is secreted in the linings of intestinal and respiratory tract, mainly found extravascularly.

It mediates allergic reactions by releasing histamine from mast cells after binding to the antigens. It also helps in eradicating parasitic infection.