B Cells and Antibodies Flashcards
why are antibodies considered a dimer of dimers
two heavy chains that are the same, two light chains that are the same
light chains are shorter, only found in Fab region that binds antibody
both binding sites (each extension of the Y shape) are the same
which of these is true:
a. different antibodies can bind to different epitopes on the same antigen
b. different antibodies can bind to the same epitope on an antigen
both true!
part of the antibody that contacts that antigen
CDRs (complementary determining regions)
region that shows great variability to increase immune diversity (via VDJ recombination)
____ portion of antibodies controles effector functions such as complement binding
Fc portion - various isotypes that can be changed via class switching (IgM/G/A/E/D)
*note that during isotype switching, only Fc (NOT Fab region) changes
what are the isotypes/classes of antibodies and their specialization?
IgM: main antibody in primary response, BEST at fixing complement, monomer form is naive BCR (IgD), pentamer held by J chain
IgG: main blood antibody in secondary response, neutralizes toxins/opsonization/fixes complement, CAN cross placenta, binds phagocytes
IgA: secreted in mucus/tears/saliva, can be monomer, highly abundant in gut/mucosal surfaces
IgE: allergy and anti-parasitic responses, binds mast cells and basophils
IgD: naive BCR (function unknown)
T/F: each B cell expresses only one BCR specificity
TRUE
4 ways antibodies cause humoral immune response:
- neutralization: bind pathogen or toxin and prevent its effect
- opsonization: antibodies coat antigen, induce phagocytosis
- ADCC: antibody-dependent cellular cytotoxicity
- complement activation
opsonization works by antibodies binding ____ on phagocytes
antibodies coat antigen
antibodies bind Fc receptors on phagocytes
phagocytosis is induced
IgG1 and IgG3 are very good at
opsonization
IgM and IgG1/3 are really good at
activating complement system
____ antibodies are really good at inducing activation of mast cells
IgE: involved in allergy and anti parasitic responses
Fc binds mast cells and basophils
shaped like IgG but more flexible
which Ig isotype can cross the placenta? what are other features of this class?
IgG: CAN cross placenta, very good at activating complement system
main blood antibody of secondary response, Fc binds phagocytes
5 subtypes
which type of antibody is found in gut and mucosal surfaces
IgA: secreted in mucus, tears, saliva
can be a monomer
has secretary component secreted by epithelial cells that allows transport across epithelial cells in the gut
which antibody class is best for activating the complement system?
IgM: main antibody in primary response
monomer is naive BCR (IgD)
J chain holds pentamer together
which occurs first in B cell development, heavy or light chain recombination?
heavy chain recombination,
then light chain recombination
2 checkpoints during BCR assembly occur during after IgH gene rearrangement and IgL gene rearrangement
immature B cells all begin with this type of antibody on its surface
IgM
class switching occurs later
(also all have IgD on surface, function unknown)
receptor editing in BCR development
in bone marrow, BCR that react too strongly to self are signaled for apoptosis
HOWEVER, they are given a “second chance” by another round of light chain rearrangement
if this succeeds, B cell moves to periphery and matures
**note that sometimes self-reactive BCR will sneak through, and peripheral tolerance is required to catch these (either apoptosis or anergy ensues)
what happens to mature B cells that recognize self antigen with high affinity in peripheral tissues in absence of Th?
peripheral tolerance mechanisms shut them down, either through:
a. apoptosis
b. anergy (functionally inactive)
c. inhibitory regulation, such as by CD22
what are the 2 types of B cell activation? and the 2 types of B cells involved in these?
- CD4 T cell/thymus dependent: Follicular B cells responds to protein antigens
- T/thymus independent: Marginal zone B cells respond to microbial constitutes (like bacterial polysaccharides) antigens - antigens usually contain repetitive patterns that cross link BCR
Follicular vs marginal zone B cells
follicular: CD4 T cell dependent, bind protein antigens
marginal zone: T cell independent, bind microbial antigens with repetitive patterns that cross link BCR
for follicular B cells (T-dependent), activation requires 2 signals:
- cross linking of BCR - either directly (polyvalent antigen), indirectly (APC), or through alternative complement pathway
- CD4 T cell help via CD40-CD40L interaction (essential), mediated by MHC II APC
what are the 3 ways in which T-dependent (follicular) B cells can cross link BCR? (first required signal for activation)
- directly - polyvalent antigens (+1 copy of specific epitope)
- APC presents antigen - monovalent antigens
- alternative complement pathway
4 consequences of BCR cross-linking? (first signal of T-dependent/follicular B cell activation)
- increased survival/ proliferation
- increased B7 expression (promotes T cell interaction)
- increased expression of cytokine receptors (promotes T cell interaction)
- increased expression of CCR7 and migration from follicle to T cell areas
describe signal 2 of T-dependent (follicular) B cell activation
signal 1 is BCR cross-linking
signal 2 is CD40-CD40L interaction between CD4 T cell and B cell (MHC II mediated)
only get B cell activation when you have T cell specific for the same antigen
TFH (T follicular helper T cells): special subset of CD4 Th that provide second signal to follicular B cells
describe 1st and 2nd phase of follicular (T-dependent) B cell response
1st: activation, proliferation, burst of low affinity Ag-specific Ab —> plasma cells develop that secrete Ab (mostly IgM, reside in lymphoid tissues 3-5 days)
2nd: activated B cells traffic into lymphoid follicles and form germinal centers —> isotype switching, affinity maturation —> produce long-lived plasma cells and memory B cells
germinal center response
second phase of follicular (T-dependent) B cell activation, when activated B cells traffic into lymphoid follicles and form germinal centers
outcomes: affinity maturation (high affinity antibodies, aka somatic hypermutation), isotype switching, generation of memory B cells, long-lived plasma cells (migrate to bone marrow)
how does affinity maturation of B cells occur?
aka somatic hypermutation: during germinal center reaction, somatic mutation of Ig V genes and selection of mutated B cells with high-affinity antigen receptors results in production of high affinity antibodies
mutations inserted in part of antibody that directly contacts antigen
what 4 tests do follicular B cells need to pass in the germinal centers during affinity maturation?
- take up sufficient antigen from follicular DC
- process/present antigen in context of MHC II
- efficiently interact with T follicular Th cells
- receive signals from T cells that allow high affinity B cell survival
what is the role of AID and mutator in follicular B cell affinity maturation?
AID (activation-induced deaminase): necessary for both isotype switching and affinity maturation
mutator: somatic hypermutation causes development of high affinity B cells (turned on in B cells by CD40 ligation)
which types of B cells can undergo isotype switching and affinity maturation
Follicular (T-dependent)
Marginal zone B cells do NOT undergo isotype switching or affinity maturation - mainly IgM, short-lived
what 2 signals are required for marginal zone (T-independent) B cell activation
usually respond to polyvalent antigens that can directly cross-link BCR
- BCR
- TLR activation
which respond first to infection, marginal zone or follicular B cells?
marginal zone (T-independent) occurs first, followed by follicular B cells
later follicular B cells that have undergone germinal center response take over
what changes from primary to secondary antibody response?
- larger peak response
- more IgG than IgM
- higher affinity
- only protein antigens can induce secondary response (because now follicular B cells are leading response)
match with antibody response:
bacterial toxins
bacteria in extracellular space
bacteria in plasma
with
complement activation
neutralization
opsonization
bacterial toxins —> neutralization
bacteria in extracellular space —> opsonization
bacteria in plasma —> complement activation
affinity vs avidity
affinity: strength of SINGLE antibody-antigen interaction
—> example: each IgG typically has high affinity for target
avidity: strength of ALL interactions combined
—> example: IgM has low affinity binding but 10 of them has high overall strength of binding
isotype vs allotype vs idiotype
isotype: classes of antibodies, different constant regions encoded by genes
allotype: allelic differences in antibodies
idiotype: differences in the variable region of antibodies that will ALWAYS vary from individual to individual and antibody to antibody
why are allotype and idiotype differences relevant in monoclonal antibody treatment?
immune system will recognize allotypic and idiotypic differences and may try to clear the therapeutic mAbs
when an immune response occurs in a lymph node, dividing B cells are located in:
germinal centers
antibody classes differ in their
heavy chain CONSTANT regions
in which of these will all immunoglobulin molecules be identical?
a. IgG from a single person
b. Fc fragments of IgG form a single person
c. IgG produced by a single plasma cell
c. IgG produced by a single plasma cell —> has undergone somatic recombination (so permanent point mutations)
Ig___ is present in colostrum
IgA
Ig___ would be produced in Peyer’s Patches
Peyer’s Patches are in small intestine
IgA is produced there
antibody with highest concentration in the plasma
IgG
Ig__ is synthesized by the fetus in response to in utero infection
IgM
synthesized by fetus
Ig__ is most effect class for agglutination
IgM
antibodies to ABO antigens are in Ig__ class
IgM
plasma antibody that can both opsonize and activate complement
IgG
pathogen-specific antibodies of Ig__ indicate recent infection
IgM (primary response - no class switching yet)
Ig__ contains ten identical light chains
IgM - pentamer
each molecule of Ig__ contains 4 identical antigen binding sites
IgA
what antibody class is found most abundant in breast milk?
IgA - found in secretions
what are the steps of ELISA assay? (what is specifically added when)
- coat wells with ANTIGEN OF INTEREST
- add PATIENT SERUM
- add anti-human Ab
Ig___ binds intact antigen
IgM
FcR for IgG:
a. binds intact antigen
b. transduces signal in T-cell antigen receptor complex
c. aids phagocytosis
c. aids phagocytosis - phagocytes have receptors for Fc of antibodies
