A serological study of immunoglobulin-forming cells of the mouse, normal and malignant, shows that they lack all known surface differentiation antigens of the thymocyte-lymphocyte axis: TL, θ, Ly-A, Ly-B, and MSLA. Two systems of normal alloantigens are expressed on these cells, H-2 and a new system named PC.

The gene Pca (Plasma cell antigen) which specifies PC.1 alloantigen segregates as a mendelian dominant not closely linked with H-2. This cell surface antigen is absent from thymocytes, leukemias, and very probably from thymus-derived lymphocytes also; it is present on cells of the liver, kidney, brain, and lymph nodes as well as on hemolytic plaque-forming cells of the spleen, and on myelomas. So PC.1 is properly classified as a differentiation alloantigen. The strain distribution of PC.1 does not conform to that of any known immunoglobulin allotype or cell surface alloantigen previously described.

Thus the cell surface antigens of immunoglobulin-producing cells are clearly different from those of cells belonging to the thymocyte-lymphocyte axis. Each family of cells has distinctive alloantigens, and the two families share alloantigens of only one known system, H-2. This implies that either immunoglobulin-producing cells are not derived from thymic lymphocytes, or if they are, the program responsible for the transition must include extensive revision of cell surface structure.

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