Anti-C4 in the serum of a transfused C4-deficient patient with systemic lupus erythematosus.

A C4-deficient patient with systemic lupus erythematosus had been transfused on several occasions. His red cells reacted with a proportion of anti-Rg (Rodgers) and anti-Ch (Chido) reagents, but this was due to a separable antibody that did not have anti-Rg or anti-Ch specificity. Eluates of anti-Rg and anti-Ch indicate his red cell phenotype to be Rg-Ch-. Anti-C4 has been identified in his serum that exhibits neither anti-Rg nor anti-Ch specificity, but has similar serological characteristics in reacting with C4- and C4d-coated red cells.