The adaptive immune system

Question 1

What is an adaptive immune response?

Answer 1

Specific recognition of pathogens forming memory, through activation of antigen specific lymphocytes

Question 2

How is the adaptive response escalated?

Answer 2

On activation of the lymphocyte receptor, the cells divide to produce daughter cells expressing the same receptor

Question 3

Why are lymphocytes so diverse?

Answer 3

Due to somatic recombination of the receptor coding region in the germ line cell. BCRs can also hypermutate.

Question 4

Why do TCRs require APC to break down the antigen before presenting?

Answer 4

T cells can only recognise short linear AA sequences at its single antibody domain.

Question 5

How do APCs present antigens to T cells?

Answer 5

Via MHC or HLA complexes.

Question 6

What do CD8 T cells recognise?

Answer 6

MHC I / HLA A, B, C - recognise shape and peptides from endogenous proteins.
Induces apoptosis of infected cell, followed by phagocytosis by macrophage

Question 7

What do CD4 T cells recognise?

Answer 7

MHC II / HLA DR, DP, DQ - recognise pattern. Can bind longer exogenous peptides.

Question 8

What do B lymphocytes recognise?

Answer 8

Can recognise whole antigens by binding to cell surface molecules or patterns.

Question 9

What is an antibody / immunoglobulin?

Answer 9

A secreted B cell receptor from a terminally differentiated plasma cell. Effective at presenting antigens to T cells.

Question 10

What proteins are involved in processing antigens of MHC I?

Answer 10

TAP protein - controls movement into ER
Calnexin and Calreticulin - stabilise MHC folding
Tapasin - Retain pool of MHC in ER for rapid increase in surface peptides on T cell binding

Question 11

Which cells are most likely to present MHC II?

Answer 11

DC, monocytes/ macrophages and B cells as they can internalise exogenous proteins and process them in ER.

Question 12

What is CD4?

Answer 12

A coreceptor of TCR

Question 13

What determines the function of a T cell?

Answer 13

cytokine environment

Question 14

How is a full immune response achieved and why is it important?

Answer 14

A full antibody response can only occur if both T and B cells recognise the antigen via their independent mechanisms to protect from AI.

Question 15

Where is a B cell developed?

Answer 15

Firstly within the bone marrow and then within secondary lymphoid organs on antigen exposure.

Question 16

What are the bonds linking heavy and light chains of BCR?

Answer 16

Disulphide bond

Question 17

What is the Fab fragment?

Answer 17

Fragment of antigen binding. Composed of heavy and light chain. Highly variable region that determines binding specificity.

Question 18

What is the Fc fragment?

Answer 18

The area containing the constant heavy chain which is accessible to effector ligands following formation of antigen complexes.

Question 19

What is the difference between a naive B cell and a plasma cell?

Answer 19

A naive cell has had no antigen exposure and uses its antibody as the receptor for an antigen.
A plamsa cell is an effector molecule that sheds Ig in response to an antigen

Question 20

Ig of a naive B cell cannot signal itself because its cytoplasmic tail is too short. What accessory molecule is involved?

Answer 20

CD19

Question 21

Where is the variability of a BCR present?

Answer 21

Variable patterns of AA are present with V region but concentrated into the CDR of this region.

Question 22

Which chains form the constant regions of both heavy and light chains?

Answer 22

Light chain constant sequence - Kappa or lambda
Heavy chain constant sequence - determines the type and function of the antibody. Mu = M, Gamma = G, Delta = D, Epsilon = E, Alpha = A

Question 23

What is the structure of a TCR?

Answer 23

Contains two non-identical polypeptide chains, that both have a constant and variable region linked by disulphide bond, forming the antigen recognition site.

Question 24

What is the accessory molecule of TCR?

Answer 24

CD3

Question 25

Which chains form the two types of TCR?

Answer 25

Alpha + beta is most common

Lambda and delta

Question 26

Why is somatic recombination important?

Answer 26

Allows diversity in lymphoctyes to evolve with continuously changing pathogens.

Question 27

What genes make up the genetic segments of a receptor?

Answer 27

Variable domain
Diversity genes
Joining genes
Constant genes

Question 28

Which enzyme is involved in gene recombination?

Answer 28

RAG (recombinase activating genes) 1 and 2. Ensures rearrangement in a specific order

Question 29

What condition results from absence of RAG?

Answer 29

SCID as B and T cells cannot develop. An early BM transplant is curative.

Question 30

What is the purpose of allelic exclusion?

Answer 30

An antigen receptor can only express a single specificity, but during recombination both chromosomes can be recombined to produce different specificity. Once one chromosome has successfully rearranged, the second chromosome will be prevented.

Question 31

How does a virus interfere with antigen processing?

Answer 31

Can block protein synthesis of MHC I and can block TAP to prevent formation and evade detection.

Question 32

How do NK cells detect suppression of the immune system by an antigen?

Answer 32

Normally NK bind to HLA-E to inhibit its activation. A virus that blocks MHC synthesis will downregulate HLA-E as the peptides from specific MHC are no longer available to bind to HLA-E and allow surface expression. If no HLA-E is present, NK cells are active and the infected cell is killed by cytotoxic products.

Question 33

What is the inheritance of MHC genes?

Answer 33

Co-dominant = allows variation in offspring

Question 34

How are lipids presented to T cells?

Answer 34

DC and monocytes express CD1 that can bind lipids on mycobacteria. CD1/lipid complex is presented to T cells to activate macrophages.

Question 35

What are the consequences of polymorphism?

Answer 35

Diverse antigen responses in an outbred population, binding different spectrum of peptides.
Rapidly mutating pathogens can escape MHC molecules as some are ineffective.