Regulation of B lymphocyte responses to foreign and self-antigens by the CD19/CD21 complex

Annu Rev Immunol. 2000:18:393-422. doi: 10.1146/annurev.immunol.18.1.393.


The membrane protein complex CD19/CD21 couples the innate immune recognition of microbial antigens by the complement system to the activation of B cells. CD21 binds the C3d fragment of activated C3 that becomes covalently attached to targets of complement activation, and CD19 co-stimulates signaling through the antigen receptor, membrane immunoglobulin. CD21 is also expressed by follicular dendritic cells and mediates the long-term retention of antigen that is required for the maintenance of memory B cells. Understanding of the biology of this receptor complex has been enriched by analyses of genetically modified mice; these analyses have uncovered roles not only in positive responses to foreign antigens, but also in the development of tolerance to self-antigens. Studies of signal transduction have begun to determine the basis for the coreceptor activities of CD19. The integration of innate and adaptive immune recognition at this molecular site on the B cell guides the appropriate selection of antigen by adaptive immunity and emphasizes the importance of this coreceptor complex.

Publication types

  • Review

MeSH terms

  • Animals
  • Antigens, CD / immunology
  • Antigens, CD19 / immunology*
  • Antigens, Differentiation, B-Lymphocyte / immunology
  • B-Lymphocytes / immunology*
  • Cell Adhesion Molecules*
  • Humans
  • Lectins*
  • Mice
  • Receptors, Complement 3d / immunology*
  • Receptors, IgG / immunology
  • Sialic Acid Binding Ig-like Lectin 2
  • Signal Transduction / immunology*


  • Antigens, CD
  • Antigens, CD19
  • Antigens, Differentiation, B-Lymphocyte
  • CD22 protein, human
  • Cd22 protein, mouse
  • Cell Adhesion Molecules
  • Fc gamma receptor IIB
  • Lectins
  • Receptors, Complement 3d
  • Receptors, IgG
  • Sialic Acid Binding Ig-like Lectin 2