Rabbit IgG antibodies against cord red blood cell membranes bind to complement receptor 1 (CD35)

We have previously shown that a subpopulation of cord/fetal red blood cells (RBC) binds rabbit IgG antibodies raised against cord RBC and absorbed on adult RBC (F-IgG), while control IgG, raised against and absorbed on adult RBC (A-IgG), fails to do so. In the present study, F-IgG maintained its bin ding to cord RBC surface antigens following absorption on spectrin but not after absorption on skeleton-stripped RBC membranes. Spectrin-absorbed F-Ig G-but not A-IgG-affinity-purified material from cord RBC contained polypept ides with apparent MW of complement receptor 1 (CR1) allotypes, Moreover, o n immunoblotting these polypeptides reacted with I-125-F-IgG as well as wit h I-125-anti-CR1 mAb, and binding of I-125-anti-CR1 mAb was inhibited by un labelled F-IgG, In addition, cord RBC incubated with F-IgG prior to reactio n with anti-CR1 showed decreased fluorescence intensity on flow cytometry, Taken together the results suggest that F-IgG binds to CR1 which shows incr eased expression/accessibility on a subpopulation of cord/fetal RBC.