Rabbit immunoglobulin gamma (IgG) was digested with plasmin after being left for 15 min at pH2.5, 30 degrees C followed by a rapid increase in the pH to 7. The fragment antigen and complement binding (Facb) was isolated and characterized chemically and biologically. Sequence studies showed that the C-terminal quarter of the heavy chain had been removed, the split occurring at a lysine-alanine bond in the sequence Thr-Ile-Ser-Lys-Ala-Arg. The fragment Facb retained the capacity to precipitate with antigen and the precipitate caused activation of the first component of complement of the same order as that of acid-treated IgG. Both Facb and acid-treated IgG showed a fall in complement fixation relative to the native molecule of 30-40%.

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