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ANTIGENS      Antigens are any substances that are capable, under appropriate conditions, of inducing the formation of antibodies and reacting specifically with the antibodies so produced. They react with both T-cell recognition receptors and with antibodies. These antigenic molecules may have several antigenic determinants, called epitopes, and each epitope can bind with a specifi c antibody. Thus, a single antigen can bind to many different antibodies with different binding sites.      Some low-molecular-weight molecules called haptens are unable to evoke an immune response but can react with existing antibodies. These molecules need to be coupled to a carrier molecule to be antigenic.      For some molecules such as drugs, the molecule needs to be conjugated to a carrier. The carrier may be a host protein. The tertiary structure of the molecule as well as the amino acid sequence is important in determining antigenicity. ...

INTRODUCTION      It is generally believed that the immune system evolved as the host’s defense against infectious agents, and it is well known that patients with deficiencies in the immune system generally succumb to these infectious diseases. However, as we shall see, it may well play a larger role in the elimination of other foreign substances, including tumor antigens or cells and antibodies that attack self. An immune response may be conveniently divided into two parts:  1) a specific response to a given antigen and  2) a more nonspecific augmentation to that response.       An important feature of the specific response is that there is a quicker response to the antigen during a second exposure to that antigen. It is the memory of the initial response that provides the booster effect. For convenience, the specific immune response may be divided into two parts:  1) the humoral response and  2) ...