The cluster of differentiation (CD) system is commonly used as cell markers in immunophynotyping. Different kinds of cells in the immune system can be identified through the surface CD molecules which associating with the immune function of the cell. There are more than 32 CD unique clusters and subclusters have been identified. Some of the CD molecules serve as receptors or ligands important to the cell through initiating a signal cascade which then alter the behavior of the cell. Some CD proteins do not take part in cell signal process but have other functions such as cell adhesion. CD18, also known as RP15, is a B-cell surface molecule belonging to the family of pathogen receptors, Toll-like receptors (TLR). CD18 has an extracellular leucine-rich repeats and a short cytoplasmic tail. CD18 / RP15 interact with an extracellular molecule named MD1 and then together form the cell surface receptor complex RP15 / MD1 which induces B-cell activation in humans and mice, leading to proliferation and up-regulation of a costimulatory molecule, B7.2 / CD86. CD18 / RP15 also has a role in LPS response because B cells lacking RP15 show hyporesponsiveness to LPS.