The lymphoid cell line differentiates into two types of lymphocytes, the B lymphocytes and the T lymphocytes. These cells constitute only about 20% to 45% of blood leukocytes. They are small cells, only slightly larger than an erythrocyte, but B and T cells can be identified microscopically by large nuclei that occupy most of the cytoplasmic volume. The nuclei are large to contain enough DNA to enable the T and B cells to biosynthesize massive amounts of protein needed to carry out their immune functions. T lymphocytes are involved in cell-mediated immunity (CMI); B lymphocytes differentiate into Ab-producing plasma cells. B lymphocytes express antibodies on their surfaces that bind antigens. T lymphocytes express specialized T-cell receptors on their surfaces that bind major histocompatibility complex 1 (MHC-I) and 2 (MHC-II) complexed with anti-genic peptide fragments.
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