Chapter 6: Lymphocyte Cell Receptors

Question 1

Define antigen receptor

Answer 1

B and T cell receptors which recognize different antigen epitopes

Question 2

Define immunoglobulin

Answer 2

Secreted antibody/ antibody component off cellular receptor

Question 3

Define Fab

Answer 3

Portion of the antibody which binds to the antigen (“Antigen binding fragment”)

Question 4

Define Fc domain

Answer 4

Cellular binding fragment (Crystallizable fragment) allows binding to host tissues, immune cells, and complements

Question 5

Define hypervariable region

Answer 5

Also referred to as complementarity determining regions. Participate in antigen binding by forming the region complementary in structure to the antigen epitope

Question 6

Define combinatorial diversity

Answer 6

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)

Question 7

Define junctional diversity

Answer 7

1. 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
2. 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

Question 8

V(D)J recombinase (RAG1/2) function

Answer 8

This enzyme removes introns and some exons from the DNA and splice segments into functional Ig genes during B-cell development

Question 9

TdT function

Answer 9

Adds around 20 non-template nucleotides randomly to the ends of the coding section, adding randomness to the junctions between segments

Question 10

General function of an antigen receptor

Answer 10

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

Question 11

What kind of molecule serves as the antigen receptor for a B cell?

Answer 11

Membrane-bound antibody (IgM or IgD) with Igå and Ig∫ for signal transmission (ITAMs)

Question 12

How many total polypeptides make up an antibody protein?

Answer 12

Four: two light chains and two heavy chains

Question 13

What is the role of the variable region?

Answer 13

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.

Question 14

What’s the function of the constant region?

Answer 14

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

Question 15

Methods of B-cell receptor/ antibody diversity

Answer 15

1. Combinatorial diversity
2. Junctional and insertional diversity
3. Somatic hypermutation

Question 16

Calculations for combinatorial diversity

Answer 16

1. Number of possible different receptors (for heavy chain): number of V genes x number of D genes x J genes
2. Number of possible receptors (light chain): # V genes x # J genes
3. Total antibody (receptor) diversity= # of heavy chains x number of k light chains + # of heavy chains x # of light ¥ chains

Question 17

How does rearrangement occur?

Answer 17

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

Question 18

Rules for gene rearrangements

Answer 18

1. Rearrangements only occur between segments on the same chromosome
2. Heptamer must pair with complementary heptamer (same for nonamer)
3. One of the RSSs must have a spacer with 12 base pairs and the other must have the 23-bp spacer

Question 19

Steps of rearrangement

Answer 19

1. Synapsis (RAG1/2 bind and hold two coding regions close together)
2. Cleavage (RAG 1/2 nicks single strand at 5’ border of heptamer bordering V and J segment (in this example))
3. 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)
4. Hairpin cleavage (Artemis endonuclease oopens hair pins at ends of V and J regions, reproducing uneven overhand)
5. Overhang extension (overhands are filled in with complementary palindromic (P) nucleotides at coding joint)
6. DNA ligation (light) (V and J strands are joined by DNA ligate 4)
7. 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
8. DNA ligation (heavy)

Question 20

Allelic exclusion

Answer 20

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

Question 21

T-cell receptor structure

Answer 21

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

Question 22

T-cell receptor diversity

Answer 22

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