Specific segments or functional units of Factor H are targeted in human diseases. Segment A is mainly affected in C3G and AAV. Segment B contains the AMD-relevant Y402H polymorphism. Both variations affect Factor H and FHL1. Segment C has not been associated with disease so far. Segment D is often altered in aHUS and is the major target for autoantibodies in DEAP-HUS. In addition, pathogenic microbes attach Factor H (and also FHL1) to their surface via segment B, and they also acquire Factor H and FHR1 via segment D. ND, not determined.