Density and size distributions of the intramembranous particles in the cytoplasmic membrane of human peripheral blood lymphocytes. II. Ultrastructural differences between T and B cells.

Separated T and B lymphocytes from human peripheral blood were studied using the freeze-fracture technique. Quantitative analysis performed on density and size of intramembranous particles (IMPs) present on both fracture faces of the plasma membrane has revealed remarkable differences between cells belonging to the two main lymphocyte populations. In particular: (a) both fracture faces of the cytoplasmic membrane of B lymphocytes exhibit larger particles than T lymphocytes; (b) the mean densities, on both protoplasmic (PF) and external (EF) fracture faces, in B lymphocytes are lower than in T lymphocytes; (c) in B cells the partition ratio of particles between PF and EF is reversed with respect to T cells; (d) on both fracture faces of B lymphocytes, the IMP densities present a normal distribution while on T cells, density values show bimodal distributions indicating the existence of two cell subsets differing in particle density.