Antigens common to human and bacterial cells., 3. Autoagglutinin to blood group A stimulated by sepsis due to Proteus mirabilis.

Cross-reactions between determinants common to human and bacterial cells sometimes involve antigens of ABO blood groups. In this case study, a blood group B-active Proteus mirabilis isolated from a septic female was shown capable of acquiring blood group A antigen from a pure medium enriched with group-specific erythrocyte stroma. This A-like antigen could not be removed by washing. Absorption of the patient’s serum by her infecting bacterium removed an autoantibody to blood group A and significantly decreased isoantibody to blood group 8. Although she did not demonstrate spontaneous agglutination of her own RBC, her RBC were weakly Coombs positive when the autoantibody was present. This report of autoantibody stimulated during the course of gram negative infection by interaction of infecting bacteria and autologous blood group antigens suggests the possibility of similar mechanisms during genesis of some autoimmune diseases. It further suggests that bacterial mimicry may exist when the invading organism can either produce or acquire host antigen, since some control pathogens did neither when tested against the patient’s pathogen.