Antibodies

Antibodies

Antibodies are glycoproteins involved in the function of the immune system.

They have a quaternary structure made up of four polypeptide chains; there are two heavy chains and two light chains.

The structure of an antibody is Y-shaped, as illustrated in the diagram below:

The components labelled are:

• Arms: The two branches of an antibody.  
• Variable region: An area at the end of the antibody arm that differs between types of antibody; the binding site is found here.
• Binding site: A 3D area on the variable region that is complementary to one type of antigen.
• Constant region: A structure that is the same across all antibodies; it may have a receptor to help phagocytes bind to it.
• Hinge region: A flexible part of the antibody which allows more than one pathogen to be attached to (by each arm).
• Disulfide bonds: These hold the polypeptide chains together.

There are three types of antibodies, each with a different role:

• Opsonins: Bind to antigens on pathogens to make it easier for phagocytes to engulf them by providing a binding site for phagocytosis.
• Agglutinins: Bind to antigens on different pathogens so that they are clumped together (agglutinated), allowing more to be engulfed by phagocytes.
• Anti-toxins: Bind to the toxins made by pathogens, making them inactive and neutralising their harmful effects.