This book incorporates the studies on direct effect of a particular genotype on the E-CR1 levels and Knops Blood Group Polymorphism. Complement receptor 1 (CR1, CD35, C3b/C4b receptor) is an immune regulatory molecule that is expressed on the surface of erythrocytes (RBCs), macrophages, neutrophils, lymphocytes, follicular dendritic cells and kidney podocytes. The functions of CR1 are binding to and removal of C4b and C3b bearing immune complexes from the circulation, enhancement of phagocytosis, regulation of complement activation by means of decay-accelerating activity for C3 convertases and cofactor activity for factor I, and serves as a complement regulatory protein. This book is for undergraduate and postgraduate students of Biochemistry and Immunology. It provides background theory of complement system and distribution of density and knops blood group polymorphisms of complement receptor 1 in normal healthy Indian subjects.