Spontaneous formation of immunogenic conjugates by 5-aminosalicylic acid--a biotransformation product of salicylazosulfapyridine.

5-Aminosalicylic acid (5-ASA) reacts readily with proteins at alkaline pH and forms conjugates capable to induce hapten specific antibodies in rabbits. Extensive immunological cross reactions have been found between 5-ASA and para-aminophenol (PAP) immunological systems, but no cross reactions were found between 5-ASA and orto-aminophenol (OAP) immunological systems. Moderate cross reactions were found, occasionally between 5-ASA, 3-hydroxy-sulfanilamide (3-HS) and alpha-methyldopa (alpha MD) immunological systems. Antibodies directed against normal rabbit serum (NRS) proteins, which served as carrier in the 5-ASA conjugates used for immunization, were found in the sera of three out of the six rabbits inoculated with such conjugates. Antibodies directed against heterologous proteins were found in all of these sera, including one serum which lacked hapten specific antibodies and the three sera lacking antibodies directed against NRS proteins. The significance of these findings for the interpretation of the immuno-allergic reactions induced by sulfasalazine is discussed.