Molecules that can be recognized by the immunoglobulin receptor of B cells or by the T-cell receptor when complexed with major histocompatibility complex (MHC) are called antigens. The word antigen is a shortened form of the words“antibody generator.” Antigens are substances that react with antibodies, while immunogens are molecules that induce an immune response. In most cases, antigens are immunogens, and the terms are used interchangeably. The antigens that are not immunogenic but can take part in immune reactions are termed as haptens. The term immunogenicity means the ability of an antigen to elicit an immune reaction in the form of a B-cell or T-cell response, whereas the term antigenicity means just the ability to combine specifically with the prod-ucts of the above responses. All molecules that are immuno-genic are antigenic too, but all antigenic molecules cannot be considered immunogenic. Thus, haptens can be said to lack immunogenicity.