Definition: Given an abstract of a paper, generate a title for this paper such that conveys the key focus of the paper.
Input: Abstract: The alternative or classical pathways for complement system component C3 may be triggered by microorganisms and antigen-antibody complexes. In particular, an activated fragment of C3, C3b, covalently attaches to microorganisms or antigen-antibody complexes, which in turn bind to the C3b receptor, also known as complement receptor 1. The genes encoding the proteins that constitute the C3-activating enzymes have been cloned and mapped to a "complement activation" locus in the major histocompatibility complex, and we demonstrate in this study such a locus on the long arm of chromosome 1 at band 1q32.

A complement receptor locus: genes encoding C3b/C4b receptor and C3d/Epstein-Barr virus receptor map to 1q32.