Ia antigens on non-lymphoid tissues their origins and functions.

Ia molecules are encoded in the 1 region of the mouse major histocompatibility complex (MIX) and expressed as surface antigens on certain immunologically active cells (reviewed in Ref. 1). B cells, subsets of T helper and T suppressor cells and accessory, antigen-presenting cells are Ia-positive (Ia+) while T killer cells and natural killer cells are not. This article discusses the nature and function of Ia+ cells found in non-lymphoid tissues, some of which are immunologically involved while others are not obviously so.