CD180 is a 105 kD type I membrane protein, also known as RP105 or Ly64. It is a member of the toll-like receptor family consisting of an extracellular leucine-rich repeat (LRR) and a short cytoplasmic tail. It is expressed on most B cells, monocytes/macrophages, and dendritic cells. CD180 associates with MD-1 to form the RP105/MD-1 surface receptor. CD180/MD-1, by working in concert with TLR4, controls B cell recognition and signalling of bacterial LPS. The ligation of RP105 by RP/14 leads to a strong B cell proliferative response, as well as protection from apoptosis induced by radiation or dexamethasone. Interestingly, in some systems, ligation of RP105 leads to Fas-independent apoptosis.