B Cells Flashcards
What is the major function of B cells?
produce antibodies (immunoglobulins)
What is a surface immunoglobulin?
B cell antigen receptor that is membrane bound
What is valency?
number of antigen-binding sites within one receptor
What are isotypes?
various kinds of antibodies with different major functions
What are the chains of antibodies?
light chain, heavy chain
What does the hinge region do?
holds two heavy chains together by disulfide bonds
What are the aspects of the variable region?
tremendous variation determining antigen specificity, contribution from light and heavy chain form antigen-binding site, “basic unit” has 2 identical antigen-binding sites (bivalent)
What are the aspects of the constant region?
at C terminus, do NOT influence specificity
What is the Fc portion or fragment?
fragment crystalizable, confers important effector functions of secreted antibodies
What is the Fab’ portion?
fragment antigen binding, light chain and one region from both the variable and constant domains of the heavy chain, forms one complete antigen-binding site
How are the 5 isotypes grouped?
based on the constant region of their heavy chains
Which isotypes are subtyped?
IgG & IgA (General Authority)
Why do effector functions of isotypes differ?
they have different constant regions
Do variable regions influence the isotype?
NO
Do light chains influence the isotype?
NO
What isotype is rarely secreted?
IgD
What determines membrane or secreted forms of antibodies?
C terminus of heavy chain
What is a J chain?
disulfide bonds joins 5 IgM for a pentamer, and 2 IgA for dimer
What is the valence of IgM and IgA?
IgM = 10, IgA = 4
What antibody is part of the mucosal immune system?
dimeric IgA
What provides primary signal for B cell activation?
surface immunoglobulins with antigen bound
What is neutralization?
prevention of harmful agents interacting with host cells
What mediates neutralization?
variable regions
What is opsonization?
like complement, antibodies coat pathogen promoting phagocytosis by macrophages and neutrophils
What is antibody-dependent cell-mediated cytotoxicity?
ADCC, NK cells express Fc receptors for IgG, find cells coated with antibodies and bind/kill them
What do mast cells & basophils express?
Fc receptors for IgE, histamine and other mediators causing an allergic reaction
What antibodies are involved in neonatal immunity?
IgG crosses placenta, IgA is in breast milk; bot provide passive immunity to newborns/infants
Describe transcytosis of IgA
endothelial cells express poly-Ig receptors that bind dimeric IgA; they are endocytosed and transported through cell; in lumen IgA is released with secretory component/piece remains attached to IgA protecting it from degradation
What is the function of the secretory component/piece remaining attached to IgA
protecting it from degradation so it can survive in saliva
What is the predominate isotype in serum & lymph? mucosal secretions?
IgG, IgA
What is the first expressed isoptype?
IgM
What are naive B cells?
have not yet encountered antigen
What do mature naive B cells express on the cell surface? how are they similar and expressed?
IgM & IgD
same antigen specificity
co-expression is due to mRNA processing
What do plasma cells secrete?
pentameric IgM, rarely IgD
What do plasma cells NOT express?
surface immunoglobulins
What main antibodies do isotype switching?
IgG for systemic immunity
dimeric IgA for mucosal immunity
What isotype do plasma/memory cell express?
the resulting isotype switched isotype with the same specificity as IgM & IgD
What are T-independent antigens?
B cells responding to some antigens without interacting with helper T cells
What do T-independent antigens bypass?
the need for costimulatory signal via CD40
What antibody results from T-independent antigens?
low affinity IgM, few memory cells generated
What do helper T cells do?
provide B cells with costimulatory signal (#2) via CD40 & secrete cytokines
What do cytokines do?
promote B cell proliferation, isotype change, and memory B cell development
What responds during secondary response?
memory B cells; plasma cells secrete IgG or IgA with changing effector functions from isotype switching based on secreted antibodies
What is affinity maturation?
where antibodies gain higher affinity for antigens during the course of an immune response
What is somatic hypermutation?
variable region genes of light and heavy chains undergo point mutations, some have increased affinity
What is isotype switching? what is the outcome?
change from IgM to another isotype, only constant region of heavy chain, antigen specificity remains the same
How does isotype switching occur?
the mue switch region pairs with another switch region forming a loop which is then excised (deleted)
Why is isotype switching called permanent?
because those cells can never express IgM or IgD again
What increased diversity of the variable region?
multiple V, J and D gene segments
Where does DNA reorganize by recombination?
precursor B cells in bone marrow
How can recombination occur for light and heavy chains?
light: any V and virtually any J can recombine = one rearrangement
heavy: any V, D, and J can recombine = two rearrangements
How is the repair of gap described?
sloppy, further increased diversity
How is the diversity process described?
random and independent of antigen; the shuffling of the gene segments is the main contributor to antibody diversity
