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The F1 hybrids of NZB and several normal mouse strains are known to produce less anti-erythrocyte (Coombs) autoantibody and develop a milder hemolytic anemia than their NZB parents. We have found that serum from some (NZB x CBA)F1 mice agglutinated erythrocytes from certain Coombs-positive NZB mice, often in extremely high titer, whereas other (CBA x NZB)F1 sera agglutinated erythrocytes from different individual NZB mice. The agglutination was due to antibody, but was not due to rheumatoid factor activity. Because F(ab')2 fragments of the F1 sera agglutinated erythrocytes coated with F(ab')2 fragments of the appropriate NZB sera, the observed reactivity was probably caused by idiotype-anti-idiotype interactions. In addition, because F1 sera could not agglutinate mouse erythrocytes coated with monovalent NZB Fab' fragments, the recognized idiotype probably involved the antigen-binding site. Anti-idiotypic antibodies against anti-erythrocyte autoantibodies may play an important role in the regulation of autoantibody formation.