An Fc receptor-bearing, third population of human mononuclear cells with cytotoxic and regulatory function

In 1973 Frøland and Natvig reported that 14.5% of human blood lymphocytes expressed distinctive Fc receptors for IgG (FcR) and that this subset lacked surface receptors believed to be characteristic for T and B cells. They named this subset 'third population' cells. Although these cells have been extensively studied since that time, whether they are a single unique population or immatureforms of other mononuclear populations is controversial. Their lineage is unknown. They have morphologic featur s that distinguish them from other mononuclear cells, but studies with monoclonal antibodies have revealed that they share surface markers andfunctions with T cells, monocytes and even granulocytes. Subsets of these cells are the effectors of natural and antibody-dependent cell killing, regulators of T-cell proliferation and suppressors of immunoglobulin synthesis. Here, David Horwitz and Antony Bakke discuss the present understanding of these cells, which they call 'L cells' because of the characteristic temperature-labile attachment of IgG molecules to their Fc receptors.