General outline of B and T cell development
What can antibodies recognize?
What mediates binding?
What mediates Ab signaling function?
What mediates Ab effector functions?
Macromolecules - Conformational and linear epitopes
Antigen recognition mediated by variable regions of heavy and light chains of Ig
Signaling functions are mediated by proteins (Alpha/Beta chains) associated with membrane Ig
Effector functions are mediated by constant regions of secreted Ig
What is the structure of an antibody?
5 different classes (isotypes) that are functionally different due to constant region differences
Light chains are either kappa or lambda (not changed during class switching)
3 regions of hypervariability for heavy and light chain: Contact sites for epitope in the antigen binding site: Complementarity-determining (CD regions) CDR1,2,3
Subtypes: IgA 1,2
Heavy chain – Alpha(1,2)
Serum concentration – 3.5 mg/mL
Serum half-life – 6 days
Secreted form – Monomer, dimer, trimer
Functions – Mucosal immunity
Heavy chain – Delta
Serum concentration – Trace
Serum half-life – None
Secreted form – None
Functions – Naïve B cell antigen receptor
Heavy chain – Eta
Serum concentration – 0.05 mg/mL
Serum half-life: 2 days
Secreted form: Monomer
Functions: Mast cell activation (immediate hypersensitivity), defense against helminthic parasites
Heavy chain – Gamma (1,2,3,4)
Serum concentration – 13.5 mg/mL
Serum half-life – 23 days
Secreted form – Monomer
Functions: Opsonization, complement activation, Ab-dep cell-mediated cytotoxicity, neonatal immunity, feedback inhibition
Heavy chain – Mu
Serum concentration – 1.5 mg/mL
Serum half-life – 5 days
Secreted form – Pentamer
Functions – Naïve B cell antigen receptor, complement activation
Define the following types of isotypic, allotypic, idiotypic:
Isotypic – Large differences in constant regions which determine IgG vs IgA vs etc.
Allotypic – Small differences in constant regions which make the isotypes unique to the individual
Idiotypic – Variable region differences
How does Ig affinity change during immune response?
What is the on-rate and off-rate during immune response?
What is the role of accessory molecules?
Affinity of Igs increase during immune response
Rapid on-rate, variable off-rate
No accessory molecules
What do T-cells recognize?
How is antigen recognition mediated by variable regions of alpha/beta chains?
How are signaling functions mediated?
What effector functions do T-cells have?
Recognize peptides displayed by MHC molecules on APCs - Linear epitopes
Antigen recognition mediated by variable regions of alpha/beta chains
Signaling functions mediated by CD3 and Zeta proteins associated with TCR
TCR does not perform effector functions
How does Ag recognition change for T-cells during infection?
What is the on-rate and the off-rate?
What molecules also are needed for recognition?
No change during immune responses
Slow on-rate, slow off-rate
CD4 or CD8 simultaneously binds MHC molecules
Define affinity vs avidity?
Affinity – Specific interaction binding kinetics
Avidity – The combination of all the affinities over an entire process (multiple reactions)
What is the rearrangement sequence of the heavy chain?
VDJC is then expressed within the cell
What is the rearrangement sequence of light chain?
VJC expressed and joins heavy chain at the cell’s surface
What are the types of diversity?
Combinatorial diversity: V-(D)-J number of combinations
Junctional diversity: Post combination, endonucleases can excise portions of the combined locus creating distinct new loci
Mix and match pairing of light and heavy chains
Somatic hypermutation - Important step of Ig affinity maturation, Not strictly a method of diversity
What is allelic exclusion?
Only maternal or paternal allelic recombination occurs not both
If one succeeds the other does not occur
If neither succeeds the cell dies
Protects clonal specificity
What is the importance of the RAG gene in recombination?
Necessary for Pro lymphocyte to reach Pre lymphocyte
Deficiency results in SCID
What are the maturation steps involved for lymphocytes?
What are the checkpoints?
Proliferation – IL-7 mediated
Pre-B/T antigen receptor expression – First recombination
Failure is the first checkpoint, leads to cell death
Proliferation of Pre-cells
Antigen receptor expression – Second recombination
Failure is second checkpoint, leads to cell death
Positive and negative selection
Explain positive and negative selection, and what kind of avidity does it display?
Positive selection - Mature cells show weak self-antigen recognition (Low avidity)
Negative selection – Strong self-antigen recognition (High avidity)
Lack of positive selection – No self-antigen recognition (No avidity)
Where does T-cell maturation occur?
What CD expression is there for the immature cells?
How does positive selection effect CD?
Thymus is the major site
Cortical region – Positive selection
Medullary region – Negative selection
Immature cell are CD4/CD8 double positive
Positive selection – Self reactive MHC+ peptide live and proceed to the next stage
If Class I recognition – CD8+
If Class II recognition – CD4+
What are the specific changes in Ig DNA,RNA and Ig expression for B cell maturation?