MD1 is a 28 kDa molecule that is associated at the cell surface with RP105 (CD180). RP105 is a leucine-rich repeat (LRR) molecule that is expressed on B lymphocytes. It was first identified by a mAb that protects spleen B cells from irradiation-induced apoptosis. LRR proteins, such as Toll receptors, have a role in innate immunity. MD1 is primarily expressed by B cells, dendritic cells and monocytes, and promotes the recognition of, and subsequent signaling by LPS in the innate immune system. MD1 does not have any homology to other molecules. MD1 when expressed alone behaves as a secretory protein. Expression of MD1 in a cell increases the expression of RP105.