L5 - Humeral responses Flashcards
What triggers the activation and proliferation (clonal expansion) of B cells?
Binding of antigen to the B cell surface.
How do B lymphocytes recognise antigen?
By expressing B cell receptors (BCRs) on their surface
When antigen binds, crosslinking Ig molecules, what happens?
The B cell is activated and secretes soluble Ig in the form of antibodies.
What types of chain make up an antibody?
2 identical light chains and 2 identical heavy chains
What do they light and heavy chains have in common?
They both have one variable region
Where do B cells arise from?
Lymphoid progenitors in the bone marrow
What are the 2 loci which can encode the light chain?
Lambda and kappa loci
How many of each region are there in the kappa chain?
40V regions and 5J regions
How many of each region are there in the lambda chain
29V regions and 7J regions
What is the first step of generating diversity in light chains and how is this controlled?
Somatic recombination of gene segments (V + J) controlled by RAG (recombination activating gene products)
Put these processes in the correct order regarding generating diversity in the light chain; Splicing, transcription, somatic recombination, translation.
Somatic recombination. transcription, splicing, translation
Which extra region is involved in somatic recombination in heavy chains?
Diversity (D). Therefore there are 2 recombination steps during the generation of diversity on heavy chains
Explain ‘combinatorial diversity’.
Any light chain can combine with any heavy chain, so in total there are around 4,000,000 different specificities/structures.
Explain ‘junctional diversity’
Imprecise joining of the V (D) J segments during somatic recombination due to deletion or addition of nucleotides between the segments.
What does TdT do?
Terminal deoxynucleotidyl transferase adds nucleotides at the junctions
Which process of diversity generation does not occur in T cells?
Somatic hypermutation
Which process(es) of diversification occur in the bone marrow?
Somatic recombination; combinatorial diversity combinatorial association and junctional diversity
Which two processes of diversity generation occur in secondary lymphoid tissues?
Somatic hypermutation and affinity maturation
Explain somatic hypermutation
Mutations in V region sequences during B cell replication causing slight changes in specificity.
Which enzyme induces hypermutation?
AID activation-induced cytidine deaminase
Explain affinity maturation
The higher the affinity of the immunoglobulins for the antigen, the stronger signals the responsible B cells receive and the more they divide. So antibody affinity increases over time.
Which 2 signals are required for B cell activation by a thymus dependent antigen?
1) Antigen binding to Ig. 2) CD40L on helper T cell binding to CD40 on B cell
Which 2 signals are required for B cell activation by a thymus independent antigen? (T cell help not required)
1) Antigen binding to Ig. 2a) Antigen binding to TLR eg. LPS, a T-independent antigen type 1 (TI-1). 2b) Extensive crosslinking by polymeric antigen, a T-independent antigen type 2 (TI-2)
Although B cells can be activated without the help of T cells, what are the implications of this which limit to effectiveness of the immune response?
Little/no isotype switching
No somatic hypermutation
No affinity maturation
No memory B cells
