CD180, also known as RP105, is a 105 kD member of the toll-like receptor (TLR) family and is expressed on most B cells, monocytes/macrophages and dendritic cells. The TLR family's structural characteristics are critically involved in protein-protein signaling interactions leading to the activation of the innate immune system. Additionally, MD-1, a secretory protein, associates with CD180 (RP105) at the cell surface and this complex, in conjunction with TLR4, recognizes bacterial LPS. The human CD180 (RP105) gene maps to 5q12. The MHR73-11 antibody is able to activate B cells and to protect against irradiation- or dexamethasone-induced apoptosis.