A large proportion of the human peripheral blood lymphocytes of adults and newborns having IgD were found also to have IgM on their membranes and vice versa. A few lymphocytes had one of these classes only. IgD and IgM could be capped independently on the same cell. The possibility that IgD was acquired by a cytophilic process was excluded by the finding that IgD-bearing cells were of one light chain type only, and by the direct demonstration of reappearance of IgD on the lymphocyte membrane during incubation in an IgD-free culture medium. On the basis of these findings, it is proposed that IgD functions as a lymphocyte antigen receptor.

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