Antibody clone BU33 recognizes human complement receptor 2 (CR2). The complement pathways form a cascade of proteins with as main task to clear microbes. A key step is the activation of C3 by C3 convertase. Regulator of this important convertase is accomplishes by a number of serum proteins. Another set of membrane bound (complement) proteins regulate cellular immune responses. After activation of C3, membrane boundiC3b, C3b and C3d arise which can interact with CR2. CR2 is expressed on the surface of B-cells, follicular dendritic cells (FDC), and some T cells. Thereby bridging innate and adaptive immunity. On FDC CR2 functions as a Fc-gamma-II receptor. On B cells, CR2 enhances cell activation. Additionally, human CR2 also binds Epstein–Barr virus, CD23, and interferon-a. C3d interaction with CR2 will collect molecules such as CD19, CD81, and Lew 13. CD19 has a long intracellular tail that triggers a signaling cascade that results in cell activation and proliferation. CR2 is a surface-associated glycoprotein composed of 15 repeating 60–70 amino acid long short consensus repeats (SCRs), a - transmembrane domain, and a short cytoplasmic tail, which may transduce signals.