Q.Describe the structure of antibody.

Step 1. Antibodies (immunoglobulins) are glycoproteins produced by plasma cells (derived from activated B-lymphocytes), highly specific to a particular antigen, and produced at an extremely rapid rate — about 2,000 molecules per second by a mature plasma cell.

Step 2. Structurally, an antibody is a 'Y'-shaped molecule built from four polypeptide chains: two identical, longer heavy (H) chains and two identical, shorter light (L) chains.

Step 3. These four chains are held together by disulfide bonds (-S-S-); the region joining the stem of the 'Y' to its two arms is called the hinge.

Step 4. Each chain has two distinct regions — a variable region, which differs from antibody to antibody and gives each antibody its unique specificity, and a constant region, which does not vary. The variable regions of a paired heavy and light chain together form the antigen-binding site, or paratope.

Step 5. Because each antibody molecule has two arms, each with its own antigen-binding site, antibodies are described as bivalent — able to bind two antigen molecules at once, which is what allows them to agglutinate (clump) pathogens.

Build the antibody's structure up in layers — overall shape, the four chains, the bonds holding them together, and the variable/constant regions within each chain.

• Describing only 'two chains' instead of the correct four (two heavy, two light).
• Omitting that the variable region specifically (not the whole antibody) is what forms the antigen-binding site.