Recombinant Human CD21 Protein, CF

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Recombinant Human CD21 Protein, CF Summary

Product Specifications

>95%, by SDS-PAGE under reducing conditions and visualized by silver stain
Endotoxin Level
<0.01 EU per 1 μg of the protein by the LAL method.
Measured by its binding ability in a functional ELISA. When CD21 is coated at 5 μg/mL, complement component iC3b binds with apparent KD <10 nM.
Chinese Hamster Ovary cell line, CHO-derived human CD21 protein
Ile21-Arg971, with a C-terminal 6-His tag
Accession #
N-terminal Sequence
Predicted Molecular Mass
105 kDa
130-140 kDa, reducing conditions

Product Datasheets

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Carrier Free

What does CF mean?

CF stands for Carrier Free (CF). We typically add Bovine Serum Albumin (BSA) as a carrier protein to our recombinant proteins. Adding a carrier protein enhances protein stability, increases shelf-life, and allows the recombinant protein to be stored at a more dilute concentration. The carrier free version does not contain BSA.

What formulation is right for me?

In general, we advise purchasing the recombinant protein with BSA for use in cell or tissue culture, or as an ELISA standard. In contrast, the carrier free protein is recommended for applications, in which the presence of BSA could interfere.


Formulation Lyophilized from a 0.2 μm filtered solution in PBS.
Reconstitution Reconstitute at 500 μg/mL in PBS.
Shipping The product is shipped at ambient temperature. Upon receipt, store it immediately at the temperature recommended below.
Stability & Storage: Use a manual defrost freezer and avoid repeated freeze-thaw cycles.
  • 12 months from date of receipt, -20 to -70 °C as supplied.
  • 1 month, 2 to 8 °C under sterile conditions after reconstitution.
  • 3 months, -20 to -70 °C under sterile conditions after reconstitution.
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Reconstitution Calculator

The reconstitution calculator allows you to quickly calculate the volume of a reagent to reconstitute your vial. Simply enter the mass of reagent and the target concentration and the calculator will determine the rest.


Background: CD21

CD21, also known as complement receptor 2 (CR2), is a 145 kDa N-glycosylated member of the RCA (regulators of complement activation) family of proteins. The complement cascade plays an important role in the innate immune system through the recognition and clearance of immune complexes and foreign particles (1). Mature human CD21 contains a 951 aa extracellular domain (ECD) with fifteen tandem SCR/SUSHI repeats, a 28 aa transmembrane segment, and a 34 aa cytoplasmic tail (2, 3). Within the ECD human CD21 shares 67% aa identity with mouse and rat CD21. Human CD21 and CD35 are encoded by two separate genes, but in mouse partially homologous proteins are alternate splice forms of one gene (4). Alternate splicing of human CD21 generates isoforms with an altered SCR8 or an insertion between SCR10 and SCR11 (5). CD21 is primarily expressed on B cells, follicular dendritic cells, and T cells. A circulating soluble form of CD21 is released by proteolytic shedding from activated B cells (6, 7). CD21 binds the complement component fragments iC3b, C3d, and C3d,g (1, 8). It forms a complex with the B cell receptor-associated CD19 molecule and lowers the threshold for B cell activation (9 - 11). CD21 can also form complexes with the complement receptor CD35/CR1 (1, 10). Mice deficient in both CD21 and CD35 exhibit normal B cell development but severely compromised germinal center development, antibody production, establishment of protective microbial immunity, and B cell tolerance to self antigens (12, 13). In mice, CD21/CD35 must additionally be present on follicular dendritic cells to mount effective humoral responses and establishment of B cell memory (14). CD21 also binds the gp350 coat protein on Epstein-Barr virus and serves as an uptake receptor for viral infection of B cells (15).

  1. Roozendaal, R. and M.C. Carroll (2007) Immunol. Rev. 219:157.
  2. Weis, J.J. et al. (1988) J. Exp. Med. 167:1047.
  3. Moore, M.D. et al. (1987) Proc. Natl. Acad. Sci. 84:9194.
  4. Kurtz, C.B. et al. (1990) J. Immunol. 144:3581.
  5. Barel, M. et al. (1998) Mol. Immunol. 35:1025.
  6. Masilamani, M. et al. (2003) Eur. J. Immunol. 33:2391.
  7. Sengstake, S. et al. (2006) Int. Immunol. 18:1171.
  8. Carel, J.-C. et al. (1990) J. Biol. Chem. 265:12293.
  9. Matsumoto, A.K. et al. (1991) J. Exp. Med. 173:55.
  10. Tuveson, D.A. et al. (1991) J. Exp. Med. 173:1083.
  11. Dempsey, P.W. et al. (1996) Science 271:348.
  12. Haas, K.M. et al. (2002) Immunity 17:713.
  13. Prodeus, A.P. et al. (1998) Immunity 9:721.
  14. Rossbacher, J. et al. (2006) Eur. J. Immunol. 36:2384.
  15. Tanner, J. et al. (1987) Cell 50:203.
Entrez Gene IDs
1380 (Human); 12902 (Mouse); 289395 (Rat)
Alternate Names
C3DR; C3DRSLEB9; CD21 antigen; CD21; Complement C3d receptor; complement component (3d/Epstein Barr virus) receptor 2; complement receptor type 2; CR2; EBV receptor; Epstein-Barr virus receptor


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