title: Regulation of B lymphocyte responses to foreign and self-antigens by the CD19/CD21 complex.
abstract: 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.
authors.affiliation: Wellcome Trust Immunology Unit, Department of Medicine, School of Clinical Medicine, University of Cambridge, United Kingdom. dtf1000@cus.cam.ac.uk
authors.forename: D T
authors.initials: DT
authors.lastname: Fearon
authors.forename: M C
authors.initials: MC
authors.lastname: Carroll
mesh_terms: D000818:Animals; D015703:Antigens, CD / Q000276:immunology; D018941:Antigens, CD19 / Q000276:immunology*; D000944:Antigens, Differentiation, B-Lymphocyte / Q000276:immunology; D001402:B-Lymphocytes / Q000276:immunology*; D015815:Cell Adhesion Molecules; D006801:Humans; D037102:Lectins; D051379:Mice; D017464:Receptors, Complement 3d / Q000276:immunology*; D017452:Receptors, IgG / Q000276:immunology; D051918:Sialic Acid Binding Ig-like Lectin 2; D015398:Signal Transduction / Q000276:immunology*
chemical_list: D015703:Antigens, CD; D018941:Antigens, CD19; D000944:Antigens, Differentiation, B-Lymphocyte; C497867:CD22 protein, human; C497868:Cd22 protein, mouse; D015815:Cell Adhesion Molecules; C080571:Fc gamma receptor IIB; D037102:Lectins; D017464:Receptors, Complement 3d; D017452:Receptors, IgG; D051918:Sialic Acid Binding Ig-like Lectin 2