Immunogen sequence: RLPTCVSVFP LECPALPMIH NGHHTSENVG SIAPGLSVTY SCESGYLLVG EKIINCLSSG KWSAVPPTCE EARCKSLGRF PNGKVKEPPI LRVGVTANF. CD21 (complement receptor 2, CR2, C3D receptor, EBV receptor) binds C3 complement fragments, especially its breakdown fragme