The monoclonal antibody MHR73 reacts with human RP105 (CD180). RP105 is a type 1 transmembrane protein of 105 kDa with extracellular leucine-rich repeats (LLRs) and a short cytoplasmic tail. RP105, assigned as CD180, is similar to Drosophila Toll in the extracellular LLRs. RP105 expression is largely restricted to immune cells including CD19+ B cells and macrophages. Histological studies showed that RP105 is expressed mainly on mature B cells in mantle zones. Antibody-mediated (MHR73) cross-linking of RP105 induces resistance against irradiation-induced apoptosis, B-cell proliferation, and up-regulation of a costimulatory molecule CD80, revealing RP105 as a potent regulator of B-cell activation. In this context it is important to note that RP105 is associated with MD1, and that it mediates B-cell surface expression and LPS recognition and signalling in cooperation with TLR4. MD-1 seems to be requisite for efficient expression of RP105.