In biological situations, single epitope molecules are rarely encountered. Much more common is a complex structure, such as a foreign cell, which contains on its surface many hundreds or even thousands of antigenic determinants. Furthermore, not all of these determinants are expressed at all times on the cell surface. It is well known that many antigens are only expressed during phases of rapid cell division or under certain physical conditions. In cancer cells it is common for antigens to be expressed which are normally only encountered during fetal development (the oncofetal antigens). Therefore, if such a complex mixture of molecules is injected into an animal, each of the antigenic determinants causes the host animal to produce a specific antibody and the serum of such an animal, if examined at a later date, will contain many different types of antibody, i.e. a polyclonal antiserum. Although such sera have been examined in the past for evidence of immune response to malignancy and infection, it is clear that the very complexity of the antigen means that discovering differences, for example, between normal cells and their malignant counterparts is extremely difficult, if not impossible. There is considerable evidence from experimental work that malignant cells are capable of exciting a definite, but weak, immune response from their host and this implies that at least some antigenic determinants differ from those on the normal cell. However, if we use the monoclonal antibody technology to produce antibodies against the same complex cell surface, we can examine each antibody in turn. In this way it has been found that the great majority of determinants are shared by normal and malignant cells. However, it is possible that an antibody may be discovered which is capable of recognizing an antigen present on a malignant cell, which is not present on its normal counterpart. Subtle differences between normal and malignant cells may be identified and exploited for diagnostic and therapeutic use. However, practice has shown that although such differences may exist, and indeed be recognized, the antigens discovered may also exist on other body tissues, e.g. antibodies raised against human colorectal cancer may recognize white cells in the bone marrow as well as the malignant tumor. A similar approach can also be used in distinguishing different types of micro-organisms which cause infectious disease. The rapid and precise diagnosis of both bacterial and viral disease is thus possible.
