The search for antibodies that distinguish cancer cells from normal cells is one of the longest uninterrupted inquiries in cancer research (1). The history of this pursuit can be divided into four phases. The first, dominated by immunologists such as Witebsky and Hirszfeld, dealt mainly with the analysis of heteroimmune serum obtained from rabbits and other animals immunized with human cancer (2). The challenge, generally unmet, was to remove antibodies reactive with normal tissue antigens; a variety of absorption techniques were devised to accomplish this. Complement fixation and, later, agar gel immunoprecipitation provided the primary systems to analyze the heteroimmune sera. Although little of enduring value came from this vast effort, two useful antigens were identified - alpha fetoprotein, a serum marker for hepatoma and germ-cell tumors (3); and carcinoembryonic antigen, a serum marker for colon and other epithelial cancers (4). The second phase in this odyssey was initiated by the work of Peter Gorer, a scientist best known for his discovery of the mouse histocompatibility locus. Gorer also had an intense interest in tumor antigens, and he introduced the approaches and test systems involving cytotoxic alloantibodies prepared in inbred mice that led to the serological dissection of normal and malignant lymphoid cells and the discovery of cell surface "differentiation antigens" such as TL, Ly1, Lyt2 (CD8), Thy-1, and PCA (5), and endogenous retroviral-coded antigens such as Gross cell surface antigen, G_IX, and ML (6). The emergence of hybridoma technology (7) transformed the field of serology and opened the floodgates for identifying new cell-surface antigens in mice and humans and for analyzing the antigenic phenotype of human cancer. Although there was great hope that monoclonal antibodies (mAb) would uncover tumor-specific antigens in humans, this has not proven to be the case. Rather, experience has shown that even the most restricted tumor antigen generally turns out to be a restricted normal differentiation antigen (8). In addition to the use of polyclonal and mAb of heterologous and allogeneic origin in the search for tumor-specific antigens, there has been a sustained effort to determine whether the autologous host recognizes cancer cells. To establish as rigorous and unambiguous a serological test system as possible for this purpose, an approach called autologous typing was developed (9), initiating what can be seen as the third phase in human cancer serology. The intention of autologous typing was to restrict the analysis to autologous reagents - tumor cells, serum, and normal cells such as fibroblasts and lymphocytes from the same patient - to eliminate the contribution of alloantigens in the reactions observed and to establish tumor specificity by absorption with autologous normal cells. With the exception of leukemia cells (10), cultured tumor cell lines were required for autologous typing, and this limited analysis to tumor types that could be adapted to growth in vitro with some regularity [i.e., melanoma (11, 12), renal cancer (13), and brain cancer (14)]. The conclusion coming from the autologous typing of a large series of patients is that a small fraction of patients do develop autologous antibody with specificity for cell-surface antigens of the tumor. With few exceptions (15, 16, 17, 18), however, molecular characterization of the antigen was generally beyond reach, primarily because the antibodies were not of sufficient titer to monitor biochemical purification or cloning.

This limitation in autologous typing has been overcome by a new approach introduced by Michael Pfreundschuh and his colleagues Ugur Sahin and Ozlem Türeci at the University of Saarland (19, 20). They called their approach SEREX, for serological analysis of recombinant cDNA expression libraries of human tumors with autologous serum. In their initial application of the method, tumor antigens, such as MAGE and tyrosinase that had originally been defined as T-cell-recognized epitopes, were detected by autologous antibody. SEREX analysis has identified a series of provocative cancer antigens that have relevance to the etiology, diagnosis, and therapy of cancer. What is so encouraging about SEREX is that it provides a way to analyze the humoral immune response to intracellular cancer antigens, a generally impenetrable forest for cancer serologists in the past. The development of the SEREX technique has inaugurated the fourth phase in cancer serology, bringing with it the prospect of providing a comprehensive view of the immune recognition of human cancer.