Recognition of Invaders

Question 1

Lymphocytes

Answer 1

B cells - act outside the cell
T cells (CD4/CD8) - attack inside the cell

Question 2

Antibodies (also known as Ig)

Answer 2

Cell surface, produced from B cells to bind foreign bodies and mediate effector functions

Question 3

Structure of antibodies

Answer 3

Two Fab arms, which are variable and bind the antigen

Fc region, the constant domain

Question 4

What are antigens vs antigenic epitopes?

Answer 4

Whole molecule antibodies recognise, usually large, >4KDa, can be proteins/carbs/lipids/DNA
Antigenic epitopes - bits that antibodies recognise, smaller regions e.g. spike proteins
Many epitopes on one antigen

Question 5

Hypervariable region of antibodies?

Answer 5

Form the complementary determining region (CDR)

Question 6

Types of epitopes?

Answer 6

Linear - amino acids in sequence recognised

Conformational - amino acids brought close together by protein folding

Question 7

Affinity of antigen binding?

Answer 7

Weak, non-covalent bonds = must be close together for strong interactions
Affinity refers to just one binding site and its antigen, determined by how good the fit is

Question 8

Avidity of binding?

Answer 8

Total strength of binding - increases with number of binding sites used

Question 9

Cross-reactivity in antibodies?

Answer 9

Antibodies bind different epitopes with differing affinities i.e. one well, one less well
Useful in mutated states

Question 10

Antibody production?

Answer 10

Need 10^11 in 5L of blood for specificity
Protective level = 10ng per ml, i.e. 5000kg!
Therefore only produced in large amounts after infection, and then kept in low levels for the rest of the time

Question 11

Functions of Fc fragment?

Answer 11

Effector domain - neutralise, preventing bacterial adherence to epithelium
Opsonisation = phagocytosis
Complement activation
ADCC

Question 12

What is the humoral immune response? What are the 4 mechanisms?

Answer 12

That mediated by antibodies
Neutralisation (block pathogen binding sites, useful for viruses)
Opsonisation (flagging for phagocytosis)
Complement Activation
ADCC (extracellular pathogens)

Question 13

5 classes of antibodies?

Answer 13

IgG, IgM, IgD, IgA1, IgE

Question 14

IgG?

Answer 14

Most abundant, IgG1-4
Diffuses into extravascular sites
Involved in all four functions, mostly neutralisation
IgG3 strongly involved in complement

Question 15

IgM?

Answer 15

First one produced by B cells so found across epithelium, up to 10 binding sites
Used somewhat in neutralisation and opsonisation
Mainly involved in complement

Question 16

IgD?

Answer 16

Rare

Don’t really know what it does

Question 17

IgA?

Answer 17

Major Ig in mucosa and serum, subclasses 1 and 2
Mainly in mucosal associated lymphoid tissue (MALT)
Found as dimer in epithelium (when secretory), monomer when diffusing
Most involved in neutralisation

Question 18

IgE?

Answer 18

Produced by mast cells so involved heavily in sensitising them
Worm infections/allergy

Question 19

What form do the Igs exist in?

Answer 19

IgM and A - polymers, IgM pentameric and IgA dimeric due to cysteine residue in the constant J chain

Question 20

T cell life cycle?

Answer 20

Originate in bone marrow (common lymphoid progenitor)
Mature in thymus
Migrate to lymphoid tissues

Question 21

Thymus and aging?

Answer 21

Reduces in size with age - 30g newborns, 3g at 80

Therefore t cell populations decline over time

Question 22

Types of t-cell?

Answer 22

CD4 - helper cells, Th

CD8 - cytotoxic, Tc/CTL

Question 23

B cell antigen receptors?

Answer 23

Antibody structure - Fc regions of light and heavy chain, with CDR and signal transducing molecules (Igs) in membrane for signalling

Question 24

T cell antigen receptors? (TCR)

Answer 24

Two chains, alpha and beta, bound by disulphide bonds with carbohydrate groups.
Top variable region, lower constant region
Va and Vb have hypervariable regins with different CDRs for antigen specificity
Also have signal transducing molecules - CD3

Question 25

How do TCRs recognise antigens?

Answer 25

Antigen presented from antigen-presenting cells, via MHCs

Question 26

MHCs?

Answer 26

MHC -1 : 3 alpha chain regions, a1-3, which binds to membrane
Non-covalently linked to a 2beta chain
1 transmembrane domain
Expressed by all cells

MHC-2 : 2 alpha, 2 beta - both chains have transmembrane domains
Less expressed - e.g. in macrophages, beta cells