Antibody molecules are essentially required to carry out two principal roles in immune defence to recognise and bind to foreign material (antigen). The host needs to be able to recognise a wide variety of different structures it has been estimated that a human being is capable of producing antibodies against more than 10' different molecular structures. This is described as antibody diversity. To trigger the elimination of foreign material in molecular terms this involves the binding of certain molecules (effector molecules) to anti body coated foreign material to trigger complex elimination mechanisms, e.g. the complement system of proteins, phagocytosis by cells like neutrophils and macrophages. The effector systems are generally triggered only by antibody molecules clustered togather as on a foreign cell surface and not by free unliganded antibody.