Complement receptor type one (CR1; CD35) binds and processes C3b and C
4b opsonized immune complexes and regulates complement activation. We
have characterized the epitopes of 13 previously reported and seven ne
w MoAbs to human CR1. The MoAbs formed seven groups based on their rea
ctivity with a panel of deletion forms of CR1. Seventeen of the MoAbs
reacted with CR1 at more than one site, a consequence of its repetitiv
e sequence. All five of the MoAbs recognizing epitopes in the nearly i
dentical repeats 3, 10, and 17, as well as one MoAb which reacted with
repeats 8 or 1/2 of 9 and 15 or 1/2 of 16, blocked cofactor activity
for C3b. Knowledge of the repeats bearing the epitopes for these MoAbs
should facilitate the further characterization of CR1.