Complement deficiencies and defects Deficiencies or defects in specific complement components have been linked to specific disorders; the following are examples: Deficiency in C1, C2, C3, MBL, MBL-associated serine protease 2 (MASP-2), factor H, factor I, or complement receptor 2 (CR2): Susceptibility to recurrent bacterial infections Deficiency of C5, C9, factor B, factor D, or properdin: Susceptibility to neisserial infections Defects in C1, C4, and C5: Systemic lupus erythematosus Defects in CR2: Common variable immunodeficiency Defects of CR3: Leukocyte adhesion deficiency type 1 Mutations in the genes for factor B, factor H, factor I, membrane cofactor protein (CD46), or C3: Development of the atypical variant of hemolytic uremic syndrome Biologic activities of complement Complement components have other immune functions that are mediated by complement receptors (CRs) on various cells.