Based on these result (negative with enzymes and cord red cells), anti-Ch or anti-Rg were suspected. What is unique about the Chido/Rodgers system?

Chido (Ch) and Rodgers (Rg) are unique in that Ch and Rg antigens are high incidence antigens present on the C4d fragment of the C4A (Rodgers) and C4B (Chido) glycoproteins of the C4 complement component, whose genes are carried on chromosome #6 (as are HLA genes).

The antigens are not intrinsic to the red cell. In antigen-positive individuals, the antigens are adsorbed onto the red cells from the plasma through an attachment mechanism that remains unclear. The system contains multiple Ch and Rg antigens.

Antibodies to Ch and Rg are generally benign but may be a nuisance in serologic investigations. Rapid identification is possible using red cells coated with C4d or by inhibition with pooled plasma from antigen-positive individuals. With the latter, a limitation is that pooled plasma also contains soluble Lea and Leb antigens.