Human blood L cells are a major subset of non-T, non-B (null) cells that lack easily detectable C3 receptors, but have high affinity Fc receptors for IgG. L cells have been purified by rosetting and immunoadherence procedures and characterized by immunofluorescence, histochemical and electron microscopic techniques. Purified L cells do not react with anti-monkey thymus serum (HuTLA) and most of them do not form rosettes with sheep or mouse erythrocytes even after the lymphocytes are treated with neuraminidase. Two major subsets of L cells are described. One has low density Ia-like surface antigens and this subset can be distinguished from B cells that have high density Ia-like surface markers. The other subset reacts with anti-myeloid serum, a reagent that characteristically stains an antigen whose density progressively increases with myeloid lineage maturation. L cells do not react with peroxidase or α-naphthyl esterase. They have ultrastructural features of lymphocytes although a deep cleft in the nucleus gives some a ‘monocytoid” appearance. These findings raise the possibility that some or most L cells are neither typical lymphocytes nor monocytes but are members of a separate hematopoietic lineage that occurpy an intermediate position in phylogeny and ontogeny.

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