[Kinds of cancer-associated antigens and their properties]

Although a number of substances have been reported to be tumor markers, no substance can be completely cancer-specific, but only a cancer-associated antigen (CAA). Now, CAA is considered to be a substance by which malignancies are distinguishable by a quantitative increase or qualitative change, even though it may be present or be produced in small quantities in normal cell as well. The following is a classification of CAAs chiefly on the basis of their origins. Group 1: ectopic CAA that is produced in a large amount in cancer cell, but can also be present or be produced in small amounts in its original normal cell. Group 2: ectopic CAA that is not present in the original normal cell, but can be produced by its carcinogenesis, though it can exist or be produced in other organs as well. Group 3: heterophile CAA that is not present in the human normal cell, but is produced by its carcinogenesis, though it can exist in other species. Group 4: fetal CAA that can be expressed in the fetal stage of the normal cell and vanishes after birth, but reappears with its carcinogenesis. This group includes oncofetal proteins, and carbohydrate antigens of which most are referred to as differentiation or developmental antigens. Based on the above classification, properties of each CAA are reviewed and discussed.