Cellular responses

Question 1

What do T-cell receptors (TCRs) recognize?

Answer 1

Short peptide sequences presented by MHC molecules.

Question 2

Which cells present antigens to CD4 and CD8 T cells?

Answer 2

CD4: APCs via MHC II • CD8: all nucleated cells via MHC I.

Question 3

How do T cells differ from B cells in antigen recognition?

Answer 3

T cells require processed peptides presented on MHC; B cells bind intact antigens.

Question 4

What is the structure of a TCR?

Answer 4

One α and one β chain, each with variable (V) and constant (C) domains.

Question 5

How many TCRs does each T cell express?

Answer 5

About 30,000 identical TCRs.

Question 6

What is the purpose of CD3 and ζ chains in TCRs?

Answer 6

Transduce signals via ITAM motifs, phosphorylated by Lck/Fyn.

Question 7

How many ITAMs are present in the TCR complex?

Answer 7

10 ITAMs across CD3ε, δ, γ and ζ dimer.

Question 8

How is TCR diversity generated?

Answer 8

Through VDJ recombination, 12/23 rule, and junctional diversity in the thymus.

Question 9

Do T cells undergo somatic hypermutation?

Answer 9

No, TCR specificity is fixed after thymic development.

Question 10

What makes contact during TCR recognition of antigen?

Answer 10

Both the MHC molecule and the peptide.

Question 11

What are the key structural components of MHC class I?

Answer 11

α chain (α1–3) + β2-microglobulin (non-polymorphic).

Question 12

What genes encode MHC class I molecules?

Answer 12

HLA-A, HLA-B, HLA-C.

Question 13

What are the key structural components of MHC class II?

Answer 13

α (α1–2) and β (β1–2) chains.

Question 14

What genes encode MHC class II molecules?

Answer 14

HLA-DR, HLA-DP, HLA-DQ.

Question 15

What size peptides do MHC I and II bind?

Answer 15

MHC I: 8–10 amino acids • MHC II: 13–25 amino acids.

Question 16

What is the peptide source for MHC I?

Answer 16

Endogenous (cytosolic) proteins degraded by the proteasome.

Question 17

What transports peptides into the ER for MHC I loading?

Answer 17

TAP transporter associated with tapasin.

Question 18

What is the peptide source for MHC II?

Answer 18

Exogenous proteins from the endocytic pathway.

Question 19

What prevents MHC II from binding peptides in the ER?

Answer 19

The invariant chain (Ii), later replaced by CLIP.

Question 20

What removes CLIP from MHC II to allow peptide loading?

Answer 20

HLA-DM.

Question 21

What are the three signals required for naïve T-cell activation?

Answer 21

1: Antigen recognition • 2: Co-stimulation (CD28–B7) • 3: Cytokines.

Question 22

What is the role of CD28–B7 co-stimulation?

Answer 22

Provides signal 2, ensuring activation only when APC is activated.

Question 23

What do CD4 T cells differentiate into after activation?

Answer 23

TH1, TH2, TH17, TFH, or TREG subsets, based on cytokines.

Question 24

What is the function of CD8 T cells?

Answer 24

Kill infected or tumor cells via perforin and granzyme release.

Question 25

What is the function of the TH1 subset?

Answer 25

Activates macrophages for defense against intracellular bacteria.