Which is the most common complement protein deficiency?

Deficiency of C2 is the most common complement protein deficiency. A deficiency of C2 or C4, early components of the classical pathway, is associated with reduced resistance to infection and reduced clearance of immune complexes. This results in increased bacterial or viral infections. However, many patients have no clinical manifestations, presumably because the alternative complement pathway is adequate for the control of most infections.Ref: Robbins and Cotran Pathologic Basis of Disease; 9th edition; Page no: 238