lecture 18- antibody diversity

Clonal selection

• One B cell clone = one antibody specificity.

• The immune system can produce specific antibody against ALL antigens, when the genome only has 25000 protein coding genes in total.

Germ Line Model: (early theory)

Antibodies are inherited

Somatic Variation Theory:

• Antibodies are diversified via mutations.

Somatic Recombination [V(D)J joining]: where?

bone marrow

Somatic Recombination [V(D)J joining] ;when?

during B cell development

Somatic Recombination [V(D)J joining] how:

• RAG proteins recognize RSS, cut DNA; recombinase enzymes stitch it.

  • Light chain: two possible genes (kappa or lambda)

    • 1 gene - V + J segments

    • 1 gene for constant C

  • Heavy chain: 

    • 1 gene - V + D+ J

    • 1 gene for constant C

    • 

VH + VL pairing

• Any heavy chain can theoretically pair with any light chain.

• Potential combinations:  ~3.4 million antibodies

  • Reality: only functional pairings survive development. Not all combos lead to functional genes

Junction flexibility (variations in final trimming)

Imprecise joining during recombination adds diversity.

• Generates different amino acid combos at the coding joint 

• P-addition: Palindromic nucleotides added at hairpins.

• N-addition: in a heavy chain, Random non templated nucleotides added by TdT enzyme.

what is Somatic hypermutation (fine tuning)

• Rapid mutation of VH and VL genes. (VARIABLE REGION) so that effects both heavy and light chain

what does somatic hypermutation do

increases affinity for antigens

where does somatic hypermutation happen ?

Germinal centers (spleen, lymph nodes).

how does Somatic hypermutation (fine tuning) happen?

• Enzyme: Activation Induced Cytidine Deaminase (AID).

what is Class Switch Recombination (CSR)

Changes the antibody class without changing antigen specificity. Affects constant regions of heavy chain Ch only

where does class switch recombo happen

germinal center

how does Class Switch Recombination (CSR) happen

• Naïve B cells express IgM and IgD.

• After activation, B cells switch to IgG, IgA, or IgE.

Enzyme: AID initiates DNA double-stranded breaks at "switch regions."

what 3 things contribute to antibody diversity

1. somatic recombination

2. VH-VL pairing

3. junctional flexibility

what two things contribute to fine tuning of antibody responce

1. somatic hypermutation

2. class switch