47 Antibody Structure basis and ontogeny of B lymphocytes

Question 1

Where does differentiation of B cells occur

Answer 1

First in liver
later fetal development and rest of life in BM

BM = primary lymphoid organ

Further differentiation can occur after migraion to lymphoid organs

Question 2

Steps of B Cell Dvelopment

Answer 2

1-4 in BM w/o specific AG
5 + 2o lymphoid organs and LN germinal centre

1. HSC commit to pro B-cell where starts to rearrange heavy chains (mainly Dh and Jh gene segments)
2. Pre B cell - rearrangement of heavy chain V, D and J gene by linking VDJ unit to Cμ > synthesis of μ polypeptide chains (if reach surface > surrogate LC)

3 Late pre-B cell - Light chain genes rearrange and light chain polypep synth. Surrogate LC synth stop and LC pair w/ μ chain > IgM momer > insert into membrane (only Ag speciic receptor i.e. IMMATURE B CELL - contact w/ AG > deletion X expansion/diff)

4. Maturation - Express IgM + D w/ identical anitigenic specificity on surface [MATURE B CELL - can respond to stimuli]
5. Ag w/ approp IgM/D > activate > prolif and diff

-> plasma cells - can secrete Ab w/ same specificity to activating Ag (early stages secrete IgM)
May undergo class switch as Ag and Th cytokines stim rearrangement - VDJ heavy chain joint to C μ and Cδ rearrange w/ other regions e.g. Y, alpha and Ɛ > diff isotype w/ same antigenic specificity 

• > Memory - activated for faster 2o response
• express isotypes other than IgM and IgD, long lived, distinct circulatory properties, High CD44 on surface
• generation assoc w/ class switch and somatic mutation in germinal centres
• germ centres allow high affinity B cells to be clonally selected via affinity maturation, low affinity undergo programmed cell death (involve activated B, Th cell, and follicular dendritic cell - present to activated B)

Self reactive Ig B cells removed via apoptosis (early stages have Ag independent)

Question 3

Genetic basis of Ab Structure

Answer 3

1. Ig act as BCR for particular epitope
2. Germ line contain multiple V, D and J segments for Ig H and L chain synth:
   Variable region of heavy - code by Vh (variable) , Dh (diversity) and Jh (joining) gene segment
   Constant region of heavy chain - code by Ch inc C mu, delta, epsilon, alpha and gamma
   Constant region of light chain - code for Cl
   Variable region light - code by Vl and Jl gene segments
3. Differentiation -
   All HC gene rearrangement > VDJ unit coding for entire variable HC region > placed next to Cμ and Cδ gene regions
   Same for LC - 1 Vl + Jl = variable region. VJ placed w/ Cl
   - if committed making K chain - VkJk + Ck gene
   - if commited to making λ - VλJλ next to Cλ gene
   After DNA rearrangement - antigenic specificity fixed (uneccesary RNA spliced out > translated into L and H chain of IGM and IgD)
4. CLass switching -
   Cμ and Cδ rearrange to join other regions e.g. CY, alpha and Ɛ > diff isotype w/ same antigenic specificity (IgA, E, G)

Diveresity

• multiple inherited genes for V region
• Rearrangement of VDJ seg and random assort of H L chains
• Junction and insertional diversity when V, D, J genes joined
• Somatic mutation - mainly after AG stim