The immune system represents the body’s main defense against substances derived from external sources (e.g., bacteria and viruses) and from abnormal processes within the body (e.g., tumor cells). These substances are collectively termed “non-self.” In order to keep the immune system in the correct range of activity, the process of non-self recognition must be highly specific and possess a memory. The specificity is conferred by peculiar receptors on the cells of the immune system able to bind foreign substances (antigens), and by the production of soluble effector proteins (antibodies) with specific antigen-recognition capacity.