Forssman (FORS) system is a new histo-blood group system with only one antigen. This Forssman (FORS1) antigen is expressed in erythrocytes, body fluids, several cells types and organs according to the species involved. Humans are Forssman negative, so the occurrence of antibodies Anti- FORS1 is highly common. However, there are individuals that are Forssman positive. Several studies suggest that the FORS1 antigen might have a key role in carcinogenesis. In this study, the aim was to determine FORS1 antigen expression in gastrointestinal cancer samples and compare the expression in normal and neoplastic tissue.

The expression of FORS1 antigen was analyzed using immunohistochemistry on gastrointestinal tumor and normal samples. The present study compared FORS1 expression in normal and tumor tissues, and the associated FORS1 expression pattern of differentiation with the therapeutic regimen.

The results demonstrated that from the 12 cases studied, 8 cases presented weak expression, 2 cases presented moderate expression of FORS1 antigen and 5 showed strong expression of FORS1 antigen in the cytoplasm of tumor cells. The results demonstrated that the intensity and extension of immunostaining differ according to the differentiation profile, suggesting that areas with a well-differentiated tumor showed higher expression of FORS1 antigen, whereas poorly differentiated areas presented less expression of FORS1 antigen. Samples from patients who performed chemotherapy regimens showed less Forssman expression, compared to patients who underwent surgery, suggesting that FORS1 antigen may have a relevant role in gastrointestinal carcinogenesis.