Focus protein: Immunoglobulin
Binding to unwanted invaders
Antibodies or immunoglobulins (Ig for short) are globular proteins, each with the same four-chain structure – two heavy and two light chains. The antibody’s four chains are arranged in a Y shape. At the end of the antibody arms are the antigen-binding sites, variable regions that differ in their amino acid sequence.
Antigens are molecules that can provoke an immune response. They are often proteins, and include parts of bacteria, viruses and cells from transplanted blood or organs. Your own cells carry ‘self-antigens’, which do not usually trigger an antibody response.
Each B lymphocyte produces a specific type of antibody. We can make as many as a hundred million antibodies, which means a hundred million different amino acid combinations. If an antibody encounters an antigen that matches it, it will bind to it, although not as precisely as an enzyme binds to its substrate. Once they are tagged by antibodies, the antigens will be destroyed by the immune system.