Chapter 6: Lymphocyte Cell Receptors Flashcards
Define antigen receptor
B and T cell receptors which recognize different antigen epitopes
Define immunoglobulin
Secreted antibody/ antibody component off cellular receptor
Define Fab
Portion of the antibody which binds to the antigen (“Antigen binding fragment”)
Define Fc domain
Cellular binding fragment (Crystallizable fragment) allows binding to host tissues, immune cells, and complements
Define hypervariable region
Also referred to as complementarity determining regions. Participate in antigen binding by forming the region complementary in structure to the antigen epitope
Define combinatorial diversity
Different V, D, and J combinations are selected randomly during lymphocyte development (and multiply) for each light and heavy chain (though only heavy chains have a D segment)
Define junctional diversity
- After RAG cleaves RSS and creates hairpin loops, Artemis asymmetrically cleaves the hairpins to make overhangs. These overhangs have complementary P nucleotides added which create unique sequences in the V-D-J junction
- With terminal deoxynucleotidyl transferase (tdt) up to 20 non-template nucleotides are randomly added to the ends of coding segments, added a string of random bases/ diversity
V(D)J recombinase (RAG1/2) function
This enzyme removes introns and some exons from the DNA and splice segments into functional Ig genes during B-cell development
TdT function
Adds around 20 non-template nucleotides randomly to the ends of the coding section, adding randomness to the junctions between segments
General function of an antigen receptor
Detect external stimuli and trigger response in the cell to which it is attached. Each receptor can recognize and bind to only one specific epitope
What kind of molecule serves as the antigen receptor for a B cell?
Membrane-bound antibody (IgM or IgD) with Igå and Ig∫ for signal transmission (ITAMs)
How many total polypeptides make up an antibody protein?
Four: two light chains and two heavy chains
What is the role of the variable region?
Makes up the antigen binding site and increases the chances that there will be some combination of aa’s that will be complementary to an antigen epitope and can bind.
What’s the function of the constant region?
For attaching to the host cells/ complement. The light chain constant can be either Kappa or Lamda (never a mixture) and binds the light chain to the heavy chain. The isotope of the heavy chain constant region determines the antibody type
Methods of B-cell receptor/ antibody diversity
- Combinatorial diversity
- Junctional and insertional diversity
- Somatic hypermutation
Calculations for combinatorial diversity
- Number of possible different receptors (for heavy chain): number of V genes x number of D genes x J genes
- Number of possible receptors (light chain): # V genes x # J genes
- Total antibody (receptor) diversity= # of heavy chains x number of k light chains + # of heavy chains x # of light ¥ chains
How does rearrangement occur?
Produced by recombination signal sequences (RSS) and located on 3’ of V segment, 5’ of J segment, and on both sides of every D segment (made of 7 nucleotide heptamer, plus spacer of either 12 or 23 nucleotides, the a 9 nucleotide nonamer)
- occurs via non-homologous DNA recombination
Rules for gene rearrangements
- Rearrangements only occur between segments on the same chromosome
- Heptamer must pair with complementary heptamer (same for nonamer)
- One of the RSSs must have a spacer with 12 base pairs and the other must have the 23-bp spacer
Steps of rearrangement
- Synapsis (RAG1/2 bind and hold two coding regions close together)
- Cleavage (RAG 1/2 nicks single strand at 5’ border of heptamer bordering V and J segment (in this example))
- Hairpin (3’ hydroxyl of coding strand attacks phosphate of opposite non-coding strand forming a hairpin and making the coding end. There’s then a blunt cut at the heptamer end, making the signal joint, which is later discarded)
- Hairpin cleavage (Artemis endonuclease oopens hair pins at ends of V and J regions, reproducing uneven overhand)
- Overhang extension (overhands are filled in with complementary palindromic (P) nucleotides at coding joint)
- DNA ligation (light) (V and J strands are joined by DNA ligate 4)
- N-nucleotide addition (heavy) ~20 non-template nucleotides are added by the TdT enzyme between the two segments, and the heavy chain segments are then ligated together
- DNA ligation (heavy)
Allelic exclusion
Ensures that each B-cell synthesizes only one heavy and one light chain. Heavy chains are recombined and expressed first, tested with a surrogate light chain, and then if it passes a light chain is produced. If one type (kappa or lamda) can’t form the right light chain, it’ll switch to the other, and if that still doesn’t work, leads to apoptosis
T-cell receptor structure
Made up of two chains, a ∫ and å chain, or ∂ and y chain, each with constant and variable regions. Is not secreted. In the complex, also has a CD3 complex of 6 protiens which signals to the interior of the cell. Also has either a CD4 or CD8 receptors to interact with MHC molecules.
- does not switch classes/ undergo affinity maturation
- single antigen binding site, not two (no avidity in one structure)
- recognizes proteins (“linear epitopes”)
- lower affinity for antigen than the antibody has
T-cell receptor diversity
Greater than antibody diversity: generated by combinatorial and junctional diversity like seen in B-cells (alpha approx light chain) through somatic recombination. Has V, D, and J segments
