Background. In 1987, three unrelated English families were reported with a putative blood subgroup called A pae. Swedish researchers later found evidence leading to abolishment of the A pae subgroup and establishment instead of the FORS blood group system (System 31 -ISBT, 2012). It is important to know the prevalence of antibodies in order to make the best decisions in transfusion medicine. Cells expressing the Forssman saccharide, such as sheep erythrocytes, are needed to detect the anti-Forssman antibody. The aim of this study was to define the prevalence of human anti-Forssman antibody. Materials and methods. Plasma samples from 800 individuals were studied. Sheep erythrocytes or Forssman "kodecytes" were mixed with the plasma samples using the tube technique. Plasma from an A pae individual was used as a negative control and monoclonal anti-Forssman antibody (M1/22.25.8HL cell line supernatant) was used as the positive control. Results. Of the 800 individuals tested, one was negative for the presence of anti-Forssman antibody. We compared the anti-Forssman antibody reaction pattern between genders and found that males have weaker reactions than females, both at room temperature (p= 0.026) and at 37 degrees C (p= 0.043). We also investigated the reaction pattern of anti-Forssman antibody in relation to ABO and Rh blood group types without finding any significant differences. Discussion. Sheep erythrocytes are suitable for searching for human anti-Forssman antibody. The quantity of anti-Forssman antibodies in plasma is higher in females than in males. In the population (n= 800) studied here, we found one individual lacking the anti-Forssman antibody. These results contribute to the data already published, confirming that FORS is a rare blood group.
