B cells Flashcards
How much of all WBCs do B cells constitute
~5%
Primary role of B cells
secrete antibodies as plasma cells
What 2 components is BCR composed of
- Antibody (Y-shaped structure) –> light & heavy chains
- Signalling transduction moiety –> IgA & IgB
BCR vs TCR -
* no. binding sites
* secreted/not secreted
BCR:
- 2 Ag binding sites
- Secreted (Ab)
TCR:
- 1 Ag binding site
- Not secreted
Major difference between BCR and secreted Ab
Secreted Abs lack a cytoplasmic tail
Antibody structure - 2 regions (+ roles)
Fab region: “for antigen binding”
Fragment crystallizable (Fc) region: interacts with Fc receptors on immune cells & recruits complement
Where is the antigen binding site of an antibody located?
between the variable region of heavy & light chains
What is the difference between the recombination of VH and VL regions
- VH - comprises 1 variable, 1 diversity, 1 joining segment
- VL - only contains 1 variable & 1 joining (lacks diversity)
Can T cells activate B cells?
Yes, T cells provide CD40 ligation (supply CD40 ligand) & cytokines
What is clonal selection?
When a B cells binds an Ag that it is specific for, it becomes activated and clonally expands
When naive B cells are activated/clonally expanded, what can they differentiate into?
- Plasma cells - secrete Abs
- Memory B cells - remember specific antigen for long time (immunological memory)
Does an Antigen’s affinity for a specific Ab increase or decrease in germinal centres?
Ags affinity for a specific Ab INCREASES in germinal centres
* Affinity maturation - “natural selection for B cells”
What is an immunoglobulin (Ig)
Antibody
Most important function of the constant region of an antibody
Dictate the isotype/class of the Ab
* and therefore determines effector functions
What is the most abundant Ab isotype
IgG - 80% in serum
First Ab isotpye to be expressed during an immune response
IgM
Avidity vs Affinity
Avidity - Combined strength of multiple binding sites
Affinity - Strength of one binding site
Does IgM or IgG have higher avidity? why?
- IgM has higher avidity due to its pentamer conformation.
- Both IgM and IgG have 2 binding sites on each Ab
- However, IgM exists as a pentamer (therefore, 5x2 = 10 binding sites), whereas IgG exists as a monomer (1x2 = 2 binding sites)
REMEMBER: Fab fragments have similar affinity, but avidity varies (depends on Ab isotype)
Remember that fucker
What region of the Ab is altered in isotype switching
Constant domain of the heavy chain
Is isotype switching reversible?
No, it is irreversible
Involves deletion (DNA recombination)
What is affected in isotype switching
Constant region of heavy chain (CH) is AFFECTED
- Variable region of Heavy chain, & light chain remain unaffected
-Therefore, Ab retains its specificity for Ag
Immune deficiency disorder where B cells cannot isotype switch
X-Linked hyper IgM syndrome –> Mostly affects males (x-linked)
- T cells do not express CD40 ligand
- Susceptible to upper respiratory tract infection
