Anti‐C3d Antiglobulin Reagents. I. Characteristics of the Anti‐C3c and Anti‐C3d Responses During Hyperimmunization in Rabbits

Antiglobulin sera designed to detect erythrocyte coating by the third component of complement (C3) should contain primarily antibody to the C3d (α2D) antigenic grouping on the C3 molecule. During immunization, animals respond independently to the C3c (βlA) and C3d antigens. The present study reports quantitative serial precipitin and antiglobulin measurements of anti‐C3c and anti‐C3d responses in 12 rabbits during a four and one‐half month hyperimmunization program and a subsequent six month follow‐up period. Anti‐C3c precipitin responses occurred earlier and were sustained longer than were anti‐C3d precipitin responses. There was a general correlation of the strength of anti‐C3d precipitin and antiglobulin responses among serum samples from a single rabbit, but not among sera from different rabbits. While most rabbits developed strong anti‐C3c and anti‐C3d precipitin and antiglobulin reactivities at the peak of their immune response, five of five commercially manufactured goat anti‐C3 sera exhibited strong anti‐C3c reactivity with little or no anti‐C3d reactivity. The implications of these observations for production of anti‐C3 antiglobulin reagents are discussed.