Several types of lymphocytes that have some features of T and B lymphocytes also function in the early defense against microbes and may be considered part of the innate immune system. A unifying characteristic of these lymphocytes is that they express somatically rear ranged antigen receptors (as do classical T and B cells), but the receptors have limited diversity.

 • As mentioned earlier, γδ T cells are present in epithelia.

 • NK-T cells express TCRs with limited diversity and surface molecules typically found on NK cells. They are present in epithelia and lymphoid organs. They recognize microbial lipids bound to a class I MHC related molecule called CD1.

• Mucosal associated invariant T (MAIT) cells express TCRs with limited diversity but do not express CD4 or CD8. They are present in mucosal tissues and are most abundant in the human liver, accounting for 20% to 40% of all T cells in that organ. Many MAIT cells are specific for bacterial vitamin B metabolites and likely contribute to innate defense against intestinal bacteria that transgress the mucosal barrier and enter the portal circulation.

• B-1 cells are a population of B lymphocytes that are found mostly in the peritoneal cavity and mucosal tissues, where they produce antibodies in response to microbes and microbial toxins that pass through the walls of the intestine. Circulating IgM antibodies found in the blood of normal individuals, even with out specific immunization, are called natural anti bodies. They are the products of B-1 cells, and many of these antibodies are specific for carbohydrates that are present in the cell walls of many bacteria and for ABO blood group antigens found on red blood cells.

• Another type of B lymphocyte, marginal-zone B cells, is present at the edges of lymphoid follicles in the spleen and other organs and also is involved in rapid antibody responses to blood-borne polysaccharide-rich microbes.

NK-T cells, MAIT cells, γδ T cells, B-1 cells, and marginal-zone B lymphocytes all respond to infections in ways that are characteristic of adaptive immunity (e.g., cytokine secretion or antibody production) but have features of innate immunity (rapid responses, limited  diversity of antigen recognition).