Antigen Recognition in Adaptive Immunity

Question 1

How do immune cells communicate?

Answer 1

1) Via soluble mediators
- accumulation of fluid, plasma proteins and granulocytes
- activation of lymphocytes
2) Cell-cell contact, mediated by ligand/receptor recognition
- recognition of target by receptors

Question 2

What are the 2 methods of recognition of target in innate immunity?

Answer 2

1) Direct recognition
2) Opsonin receptors

Question 3

What is recognition by opsonisation?

Answer 3

1) Major opsonins in plasma - immunoglobulins (Abs), complement components
2) Phagocytes express receptors for Ig Fc, i.e., they recognise the constant region of Abs, complement components C1q, C3b and C4b

Question 4

What is the difference between linear epitopes and discontinuous epitopes?

Answer 4

1) Linear - protein Ag is formed from contiguous aas
2) Discontinuous - (conformational) is formed from aas from different parts of the polypeptide that are bought together when the chain folds, i.e., are dependent on conformation of the protein

Question 5

What is the domain structure of immunoglobulins?

Answer 5

1) Basic unit of an Ig is a domain
2) CDRs
3) Hinge region
4) Ag-binding site, x2
5) Carbohydrate
6) L chains contain 1 VL domain and 1 CL domain
7) H chains contain one VH domain and 3/4 CH (depending on Ig17 class)

Question 6

What is the Ig gene superfamily - IgSF?

Answer 6

1) Genes encoding Ig domains are not restricted to Ig genes
2) Found in superfamily of related genes, particularly those encoding proteins crucial to cell-cell interactions and molecular recognition systems
3) IgSF molecules are found in most cell types

Question 7

What are 3 Ab determinants?

Answer 7

1) Isotypes - different classes of Abs due to different heavy chains
2) Allotypes - polymorphic variants found in some individuals of species
3) Idiotypes - Abs with differences in their CDRs on V regions (i.e., the Abs recognise different epitopes)

Question 8

What are framework regions?

Answer 8

1) Other regions of the V domain that are less variable
2) They do not form the Ag binding site of the Ab molecule
3) They act as scaffold that support CDR loops

Question 9

Is antibody binding is reversible?

Answer 9

Yes

Question 10

What are ADCCs?

Answer 10

1) Antibody Dependent Cell-mediated Cytotoxicity
2) Killing of antibody-coated cell

Question 11

What is Antibody-dependent enhancement (ADE)?

Answer 11

1) Aggregation of virus particles impairs infectivity
2) Interference with viral protein binding to cell receptor
3) Interference with viral protein fusion function after cell receptor binding
4) Antibody binding can enhance viral fusion

Question 12

What are the 5 immunoglobulin classes (isotypes) of H chains, differ serologically in CHO content and in size?

Answer 12

1) IgM
2) IgD
3) IgG
4) IgA
5) IgE

Question 13

What are the 2 different types of L chains?

Answer 13

1) Kappa, k
2) Lambda

A single Ig contains only 1 type (either kappa/lambda chain, never both)

Question 14

What is immunoglobulin G (IgG)?

Answer 14

1) Most abundant class in serum around 80% of total serum immunoglobulins
2) In both mouse and man there are 4 subclasses
3) IgG fixes complement
4) IgG is only immunoglobin to cross placenta
5) IgG reacts with FcR’s on phagocytic cells to promote opsonisation

Question 15

What is immunglobulin A (IgA)?

Answer 15

1) IgA is 2nd most abundant immunoglobulin in serum and most abundant immunoglobulin in external secretions
2) In serum IgA is primarily a monomer
3) In secretions IgA (= secretory IgA) is mainly a dimer

Question 16

How does secretory IgA and transcytosis work?

Answer 16

1) ‘Stalk’ of the pIgR is degraded to release IgA containing part of the pIgR - secretory component
2) IgA and pIgR are transported to apical surface in vesicles
3) B cells located in submucosa produce dimeric IgA - pIgR + IgA are internalised
4) Ig receptors are expressed on basolateral surface of epithelial cells to capture IgA produced in mucosa

Question 17

What is IgE?

Answer 17

1) Very low concentration in serum
2) Binds to Fc receptors on basophils and mast cells - induces hypersensitivity response
3) Degradation and release of granule contents
4) Histamine and other substances that mediate allergic reactions

Question 18

What is IgM?

Answer 18

1) 3rd most abundant immunoglobulin in serum
2) In monomeric form is found on surface of mature B cells together with IgD where it serves as Ag specific B cell receptor (BCR) –> after activation B cells undergo class switching to make other isotypes
3) First isotype to be produced in a primary immune response (–> elevated IgM usually indicates recent infection/ exposure to Ag)
4) First isotype to be produced by neonate and only class of Ig that is synthesised by fetus (5m of gestation)

Question 19

What is serum IgM?

Answer 19

1) Secreted by plasma cells as pentamer in which 5 monomer units are held together by disulphide bonds
2) Each pentamer contains an additional polypeptide called J (joining) chain

Question 20

How is the pentameric structure in IgM beneficial?

Answer 20

1) Has increased avidity for Ag
2) Is excluded from interstitial spaces
3) Does not cross placenta
4) Is very efficient at fixing complement

Question 21

What are B cell receptors (BCRs)?

Answer 21

1) Recognises non-self Ags
2) Activates and produces Abs (immunoglobulins)

Question 22

What are T cell receptors (TCRs)?

Answer 22

1) T cells do not recognise native Ags
2) Ag must be presented to T cells
3) Ag presenting cells (APC) process the Ag and present it into MHC
4) T cells recognise Ag with their TCR

Question 23

Why do T cells not respond to soluble Ags?

Answer 23

1) Ags must be processed and presented into MHC molecules
2) Cell surface peptides of Ag presented by cells that express MHC antigens

Question 24

What is MHC?

Answer 24

1) Major Histocompatibility Complex
2) Involved in transplant rejection
3) Most polymorphic protein in humans
4) Also named Human Leukocyte Antigen (HLA)

Question 25

How does cell activation affects level of MHC expression?

Answer 25

1) Pattern of expression reflects function of MHC molecules 2) Class I - involved in regulation of anti-viral immune response 3) Class II - involved in regulation of cells of immune system 4) Anucleate erythrocytes can't support virus replication - hence no MHC Class I, some pathogens exploit this e.g., Plasmodium species

Question 26

What are MHC Class I molecules?

Answer 26

1) MHC-encoded alpha-chain of 43kDa 2) Alpha-chain anchored to cell membrane 3) beta2-microglobulin, 12kDa, non-MHC encoded, non-transmembrane, non-covalently bound to alpha-chain 4) alpha3 domain and beta2m have structural and aa sequence homology with Ig C domains - Ig gene superfamily

Question 27

What are MHC Class II molecules?

Answer 27

1) alpha-chain of 34kDa and beta-chain 29kDa 2) alpha and beta anchored to cell membrane 3) No beta-2 microglobulin 4) alpha2 and beta2 domains have structural and aa sequence homolog with Ig C domains - Ig gene superfamily

Question 28

What do MHC Class II molecules bind to?

Answer 28

1) CD4 T cell co-receptors 2) CD4 recognises a non-polymorphic determinant on class II

Question 29

What do Class I MHC molecules bind to?

Answer 29

1) CD8 T cell co-receptors 2) CD8 recognises non-polymorphic determinant on class I

Question 30

What are properties of TCRs?

Answer 30

1) Only 1 form of TCR expressed on each T cell 2) Means each T cell and daughter cells have only 1 TCR and 1 main specificity for Ag - T cell clone 3) However, there are infinite no. of different versions of TCR each with a unique Ag binding site 4) TCR has only 1 Ag binding site 5) TCR never secreted 6) Each T cell expresses about 30,000 TCRs on its cell surface

Question 31

What is the structure of T-cell Ag receptor?

Answer 31

1) Resembles on Ig Fab fragment 2) Domain structure - Ig gene superfamily 3) Monovalent 4) Heterodimeric, chains are disulphide-bonded 5) Never secreted 6) Very short intracytoplasmic tail 7) Ag combining site made of V(alpha) and V(beta) regions 8) Ag binding site is located in variable domain on Ig and TCR

Question 32

What does the TCR four dimer complex do?

Answer 32

1) CD3(gamma), delta, episnon or zeta chains are required for cell surface expression of TcR-CD3 complex and signalling through TcR 2) Intracytoplasmic region of TcR chain is too short to transduce a signal 3) Signalling is initiated by triggering of TcR by MHC-peptide complexes on APC

Question 33

What is the process of Ag recognition by T cells?

Answer 33

1) T cells recognise Ags using TCRs 2) Recognise linear peptides - not whole proteins 3) Only recognise epitopes when they are complexed to MHC molecules 4) Therefore proteins Ags must be processed and presented by APC to be cognised by T cells 5) Process is referred to as Ag processing and presentation 6) In general CD8+ T cells recognise endogenous Ags and CD4+ T cells recognise exogenous Ags 7) MHC molecules bind multiple peptides, however, each T cell only recognises 1 peptide

Question 34

What is the process of Ag recognition by B cells (BCR -mAb)?

Answer 34

1) B cells recognise Ags using BCRs - are membrane-bound Abs (mAbs) 2) B cells recognise soluble Ags, accessible/hydrophilic epitopes 3) B cells recognise both linear/conformational epitopes, can be lost if conformational/gained if hidden core epitopes by protein denaturation 4) B cells recognise epitopes from proteins, polysaccharides, lipids, nucleic acids 5) B cells are APC (Antigen Presenting Cells) - process and present Ags to T cells