Production of antiserum to human properdin and demonstration of antigenic differences between the native and activated protein.

Antisera were raised in rabbits to human properdin in the precursor form (P) and in the activated state (P). On Ouchterlony analysis using the anti-P, reactions of complete identity were obtained between P, P, and properdin in twofold concentrated serum (NHS). However, when anti-P was used, a reaction of identity was obtained only between P and NHS, and a reaction of partial identity was formed between P and P and between properdin in NHS and P, suggestive of the fact that certain antigenic determinants in P may be lacking in P. The results indicate that activation of precursor properdin may involve proteolytic cleavage and/or conformational alterations of the molecule.