CD21 also known as complement receptor 2 (CR2)，C3d receptor or EBV receptor is a 140 kDa protein. CD21 is a glycosylated type I transmembrane protein consisting of an extracellular face of a series of 15 or 16 CCP domains. CD21 is the receptor for complement components C3d and iC3b as well as the Epstein-Barr virus (EBV) glycoprotein gp350/220. The soluble CD21 (sCD21) was shown to efficiently trigger CD23 signalling pathways in human monocytes. By inducing release of proinflammatory cytokines and upregulating expression of molecules involved in antigen presentation，sCD21 modulates critical monocyte functions that may be relevant to allergic and inflammatory disorders.