C3d antiglobulin haemagglutination of human red blood cells. A demonstration of two types of cell-bound C3d by means of typsin digestion.

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Abstract
Washed human red blood cells from blood collected in EDTA were tested by Auto-Analyzer for a percentage of maximum antiglobulin hemagglutination (AH) using monospecific antisera to human C3d [complement component 3d] and C3c. The cells from normal persons were agglutinated by anti-C3d but not by anti-C3c. To a fixed dilution of antiserum, normal C3d AH values were 34 .+-. 19% for adult cells (n = 29) and 14 .+-. 19% for cord cells (n = 19); the difference was significant (P < 0.0001). By pretreatment of these cells with trypsin the C3d AH was completely abolished or markedly reduced. Its difference between the adult and cord cells was eliminated as the observed values were 4 .+-. 7% and 3 .+-. 4%, respectively (P = 0.15). The supernatant fluid of a cell-trypsin mixture, treated with trypsin inhibitors, was inhibitory to C3d AH but not to C3c AH. The AH of C3d-coated red blood cells resulting from complement fixation in vivo (i.e., cold agglutinin disease) or in vitro (e.g. sucrose water reaction) was resistant to trypsin treatment. The difference between trypsin-sensitive and trypsin-resistant cell-bound C3d is probably at its attachment mechanism to the cell membranes. Both the advantage and limitation of using trypsinized cells for C3d antiglobulin tests are demonstrated.