Antibodies are proteins that are produced by the immune system and bind to a particular antigen that has a complementary structure. There is only a small section on the antigen molecule that is complementary to the antibody. Antigens are found on the surface of a pathogen and are different on every pathogen. Once the antibody has bound to the antigen, it inactivates it which then causes its destruction by being engulfed by a microphage.
An antibody is Y shaped and is made up of four polypeptide chains held together by disulphide bonds, two of which are heavy and two of which are light and smaller. The specificity of an antibody means that billions of different ones have to be produced so that there is the appropriate one to bind to the antigen. The antibody-antigen complexes have different strengths which are dependent on the number and affinity of the antigen binding site. Many different types of bonds such as hydrogen bonds and van der Waals forces are also important in determining how strong the interaction is between the antigen and the antibody.