Absence of streptococcal protein G (PG)‐specific determinant in the Fab region of human IgG2

It has been previously reported that CH1 Fab protein G‐contact site is responsible for the widespread recognition of mouse and human IgG Fab by PG. Here we present evidence that PG binding to F(ab′) 2 is restricted, as indicated by the lack of reactivity with PG‐Sepharose columns of a portion of F(ab′) 2 fragments obtained by pepsin digestion of human IgG from a commercial immunoglobulin preparation for intravenous use or purified from sera of two healthy blood donors and two patients with polyclonal hypergammaglobulinaemia. Isoelectric focusing showed that F(ab′) 2 fragments that did not bind PG focused in a lower pH range compared with those which did. Testing of the Fab fractions with MoAbs to κ and λ light chains or to γ1, γ2 and γ3‐Fab subclass determinants showed that γ2‐F(ab′) 2 were mainly found in the PG non‐reactive F(ab′) 2 fraction, and that this distribution was not influenced by the L chain isotype. These results indicate that the PG‐specific binding determinant(s) is not expressed in the F(ab′) 2 region of most human IgG2.