Lymphocytes are the second most numerous leukocyte (after neutrophils). The most obvious feature is a rounded nucleus that is surrounded by a rim of cytoplasm that is variable with cell size during the onset of AMI. Small, medium, and large lymphocytes can be identified by comparing their size to RBCs (~8 um in diameter). Small lymphocytes have a thin rim of cytoplasm, a heterochromatic nucleus and are about 6-9 um in diameter (HA5). These small lymphocytes can be either T cells or B cells - it is not possible to distinguish between them in routine stains.