Studies on J chain and binding site for secretory component in circulating human B cells. II. The cytoplasm.

About 0-3-1-1% of the lymphoid cells from peripheral blood of healthy adults contained cytoplasmic immunoglobulin (Ig). The class distribution of these B cells varied greatly among individuals, with a preponderance of the IgA (26-65%) or the IgG (15-66%) class. A remarkably high percentage of the Ig-containing cells were positive for cytoplasmic J chain regardless of the class (100% for IgM, 87-97% for IgA, 50-100% FOR IgD, and 43-88% for IgG cells). This feature probably reflects that the cells represent circulating blasts derived from the early expansion phase of B-cell clones. The antigenic determinants of the J chain were in most IgA-containing cells considerably masked, indicating that this subunit was "correctly" arranged in the IgA dimers at the cytoplasmic level in the manner demonstrated for intestinal IgA plasma cells.