4. Adaptive Immunity 1

Question 1

Lifespan on naive B cells

Answer 1

5 days, mature B cells have a much more longer lifespan

Question 2

Positive selection of antigen receptors

Answer 2

identifies cells that can bind antigen (in the
case of T-cells, MHC-bound peptide) and signal through their receptor
-this signal promotes their survival

Question 3

Negative Selection of antigen receptors

Answer 3

Binding of self-antigen to the B-cell receptor or T-cell
receptor, which results in deletion of the cell by apoptosis

T and B cells with high affinity for self antigen are destroyed

Question 4

Types of B cell responses

Answer 4

1) T cell dependent
- Usually proteins
- make up more of the B cell antigens

2) T cell independent
- Polysaccharides, lipids, bacteria

Question 5

How do T cell dependent antigens activate B cells?

Answer 5

2 signal model

1. Engagement of antigen receptor (BCR, “signal 1”)
2. Co-stimulatory signal (“signal 2”).

Question 6

How do T-independent antigens activate B-cells ?

Answer 6

By direct BCR (B cell receptor) aggregation

Question 7

T-cell dependent B-cell response

Answer 7

1. Antigen binding to BCR provides “Signal 1” to B-cell.
2. Antigen is internalised, processed, antigenic peptides are displayed on MHC for T-cell recognition.
3. TH (helper T-cell) recognizes antigen-MHC complex via the T-cell antigen receptor (TCR): provides “Signal 1” to T-cell.
4. CD80/CD86 on B-cell binding to CD28 on T-cell provides “Signal 2” to T-cell.
5. T-cell activation leads to up-regulation of CD40L which bind to CD40 providing “Signal 2” to B-cell.
6. Cytokine production by activated T-cell also help to activate B-cell.
7. B-cell proliferates and differentiates into antibody secreting B cell (plasma cell).

Question 8

Primary Antibody Response

Answer 8

Lag Time- 5-10 days

Peak Response
-Smaller than secondary

Antibody Isotope
-IgM>IgG

Antibody Affinity
-Lower average affinity, more variable

Question 9

Secondary antibody response

Answer 9

Lag Time
1-3 days

Peak Response
Larger than primary

Antibody Isotype
Relative increase in IgG (and IgA pr IgE)

Antibody affinity
- Affinity maturation causes higher average affinity compared to primary response

Question 10

Antigen recognition by B-cells vs T-cells

Answer 10

Both form antigen receptors by V(D)J recombination

Antigen binding
B cells- bind intact protein antigen in solution through specific binding

T-cells bind peptides displayed on APC

Question 11

How are B cells recognised immunophenotypically?

Answer 11

CD19 and CD20

Question 12

How are T cells recognised immunophenotypically?

Answer 12

CD3, CD4 or CD8

Question 13

BCR structure

Answer 13

2 light and 2 heavy chains, secreted and membrane bound

Question 14

TCR structure

Answer 14

alpha-beta heterodimer, only membrane bound

Question 15

Describe the T cell independent B cell response

Answer 15

Simple, repetitive antigens (often carbohydrates)

Mostly IgM

Modest affinity

No memory

B cells activated by direct BCR crosslinking

B cells can also be activated via Toll-like receptors (TLRs)

Question 16

Somatic hypermutation

Answer 16

Rapid single base mutations that occur in the variable region

Changes affinity of antibody

Question 17

Antibody class switching

Answer 17

B cells only

Antibody class defined by heavy chain isotype

Occurs in constant region so variable region stays the same

Question 18

Variable region

Answer 18

HC and LC

Question 19

Constant region of antibody

Answer 19

HC only

Question 20

What is the name of the enzyme that cuts the DNA in antibody class switching?

Answer 20

Cytidine deaminase