CD21, also known as complement receptor 2 (CR2), is a 145 kDa transmembrane protein that contains 15 tandem SCR/SUSHI repeats. CD21 is primarily expressed on B cells, follicular dendritic cells, and T cells. It binds the complement component fragments iC3b, C3d, and C3d,g and forms complexes with CD19 and CD35. CD21 is required for germinal center development, antibody production, and B cell tolerance. It also functions as a cellular uptake receptor for Epstein-Barr virus.