Complement is the collective designation for a complex system of sequentially interacting serum proteins that determines in large part the consequences of antigen-antibody interactions. IgG or IgM combined with antigen starts activation of the 11 proteins or 9 components that make up the classical complement reaction pathway: trimolecular C1(q,r,s), C4, C2, C3, C5, C6, C7, C8, and C9. Inherited deficiencies, genetically autosomal recessive, have been described for all these proteins except C8 and C9. In addition, genetic deficiencies of proteins that control enzymes derived from activated components have also been documented: decreased C1 inhibitor resulting in hereditary angioedema, and decreased