All clinical work utilizing MCA's to recognize tumors is based on the assumption that antigens exist on the malignant cell which are not present on its normal counterpart. It is accepted that such differences may be very subtle indeed. Much of the investigative work has been directed towards the detection of tumor-specific antigens, i.e. antigens which are present and expressed only when malignant transformation has occurred in a cell and are never found in the normal cell. So far there is no real evidence that any such tumor-specific antigen exists in human cancer although the search continues. Most work has been concerned with tumor-associated antigens, i.e. antigens which are expressed in minute quantities in the normal cell but are expresed in much greater quantity when malignant transformation has taken place. Such changes may reflect secondary characteristics of the malignant cell, such as degree of differentiation or rate of division. None the less, several of these tumor-associated antigens have been described in certain tumors and can be used for diagnostic techniques. An example is alpha-fetoprotein which is normally only detected during fetal life and in the developing liver. Its presence on hepatomas and teratomas may be explained by the increased degree of cell turnover which occurs in both embryos and tumors.
