Lecture 1

Question 1

What are the sources of infection?

Answer 1

Pathogens

Question 2

What are pathogens?

Answer 2

Organisms that cause disease
• Bacteria
• Viruses
• Fungi
• Parasites (Worms, Protozoa)

Question 3

What must the immune system be able to do for an effective immune response?

Answer 3

Be able to recognise & respond to any invading organism

Not over react to benign or self

Be able to direct different effector mechanisms against different pathogens

Question 4

What is the difference between innate & adaptive immunity?

Answer 4

Innate is activated very quickly

Adaptive takes over if innate doesn’t completely control the pathogen

Question 5

What is specific/adaptive immunity

Answer 5

Induced by exposure to a particular infection

Shows a high degree of specificity

Exhibits memory

Question 6

Features of specific immunity

Answer 6

Clonally distributed receptors

Large repertoire

Response takes time to develop

Memory cells produced

Question 7

Clonal selection

Answer 7

1. Removal of potentially self-reactive immature lymphocytes by clonal deletion
2. Pool of mature naïve lymphocytes
3. Proliferation & differentiation of activated specific lymphocytes to form a clone of effector cells

Followed by clonal expansion

Question 8

What are the 2 lymphocyte receptors for antigens?

Answer 8

B lymphocytes

T lymphocytes

Question 9

B lymphocytes

Answer 9

BCR expressed by B cells

Membrane anchored protein on the surface of B cell

Binds free antigen

Is subsequently secreted with B cell is activated = an antibody

Question 10

T lymphocytes

Answer 10

TCR expressed by T cells

Membrane form only - stays on the surface where it functions

Recognises a peptide fragment of antigen bound to MHC expressed by APCs

Question 11

What do antibodies do?

Answer 11

Activation of complement

Activation of effector cells

Question 12

What are antibodies made of

Answer 12

Formed of 4 polypeptides
• 2 heavy chains & 2 light chains

Formed by domains - variable & constant

Held together by covalent & non-covalent bonds

Question 13

Pathogens that may infect mammals can include fungi, bacteria, viruses as well as large, multi-cellular organisms such as …

Answer 13

parasites - worms, protozoa

Question 14

The first phase of an immune response is known as the innate response with the second phase called the …

Answer 14

Adaptive immunity

Question 15

The acquired phase is specific and relies on the detection of infectious agents using …

Answer 15

antibodies/cytokines specific for that pathogen

Question 16

Why is the secondary immune response more effective than the primary response?

Answer 16

The memory cells recognise the pathogen, and the antibodies can respond much faster.

Question 17

Antibody structure:

Answer 17

• immunoglobulin
• basic 4-chain structure
• 2 identical heavy chains & 2 identical light chains, held together by covalent and non-covalent bonds
• 2 types of L chain (λ & κ)
• each chain has a variable (V) and a constant (C) region
• antigen-binding sites (variable region) consist of VH and VL
• different CH regions interact with complement and the Fc region binds to different FcRs expressed by effector cells (macrophages, neutrophils, NK cells)

Question 18

What do the variable & constant regions do?

Answer 18

Variable regions form the antigen-binding sites - specific for a given Ab

Constant regions are responsible for antibody structure & interacting with other molecules

Question 19

What are the 2 types of light chain?

Answer 19

Lambda and Kappa

Question 20

5 antibody classes - what are they?

Answer 20

IgM, D, A, G and E

Isotype is determined by the heavy chain (C region)

Isotypes differ in their structure and functions

Question 21

Antibody domains

Answer 21

Analysis of amino acid sequences of H and L chains reveals homology regions (domains)
each domain ~ 110 amino acids
each domain comprises two b sheets,
linked by a disulphide bridge (S-S, C)
domains are paired –> folded units within the protein

Question 22

Immunoglobulin-like domains

Answer 22

present in a range of other proteins:
Immunoglobulin superfamily
includes:
TCR, MHC class I and II, various other molecules
(e.g. CD4, CD8, CD80, CD86, CTLA-4, KIR, IL-6R)

Question 23

Antibody – antigen interaction:

Answer 23

• variable region (heavy and light chains)
• V regions are specific for a given Ab
• concentrated region of variability
• hypervariable regions (3 in VH & VL)
• HV1-3
• 6 hypervariable loops-> Ag binding site
• complementary-determining regions (CDR1-3)
• both heavy and light chains contribute
• Ag binds to aa in CDRs
• size/shape of Ag affects binding

Question 24

Antigen recognition:

Answer 24

1. epitopes recognised by antibodies may be continuous or conformational (discontinuous)
2. Ag can be almost any molecule (protein, polysaccharide even nucleic acids), along with non-biological molecules e.g. chemicals, metals
3. antibody and Ag form non-covalent interactions
4. Ag sequence may be manipulated in vaccine design
5. Ag may be folded
6. CDRs present in antibody V regions determine the specificity and the affinity of an antibody for Ag

Question 25

What are the 2 chains of an antibody called?

Answer 25

Heavy and light chains.

Question 26

Which part of an antibody interacts with the epitope of an antigen?

Answer 26

The variable domains of the heavy and light chains (VH and VL)

Question 27

Define the Fab and the Fc regions of an antibody?

Answer 27

The Fc region is constant. The Fab region binds to antigens and consist of VH and VL

Question 28

The antigen binding site of an antibody is made up of ...

Answer 28

VH and VL

Question 29

The Lamda and Kappa chains form the L chain of an antibody?

Answer 29

Light

Question 30

How many hypervariable regions are involved with antigen binding in a single antibody molecule?

Answer 30

3 in VH and 3 in VL – 12 in total

Question 31

Antibodies only recognise peptide antigens?

Answer 31

False, antigens can be almost any molecule (protein, polysaccharide, nucleic acids, chemicals, metals)

Question 32

Lymphocyte receptors for antigen:

Answer 32

• TCR expressed by T lymphocytes: membrane form only recognizes peptide fragment of antigen bound to MHC expressed by APC

Question 33

What does a TCR do?

Answer 33

* unlike antibody, it doesn’t bind free antigen * binds/recognizes processed antigen (peptides) * “presented” in the cleft/binding “groove” of MHC class I or class II molecules

Question 34

TCR:

Answer 34

* membrane bound receptor (and unlike BCR is not secreted) * smaller than BCR/antibody (only 4 Ig-like domains) * recognizes short peptide fragments (of Ags) bound to MHC molecules on other cells (i.e. not free/ “native” antigens) * heterodimer of a and b chain (sometimes g and d) * each chain has a V and a C region * domains (x4) are Ig-like * V domains interact with antigen = peptide bound to MHC molecule * each chain contributes 3 CDRs to Ag binding

Question 35

Major histocompatibility complex (MHC) molecules

Answer 35

``` chromosome 6, 7 x106 bp MHC class I (expressed by nearly all cell types in body) MHC class II (expression restricted to a specialized group of immune cells) ```

Question 36

MHC class I and II

Answer 36

* related, but different structures * different expression patterns * present peptides from different sources * many alleles (polymorphic) * first identified due to role in transplant rejection

Question 37

MHC class I

Answer 37

• expressed on all nucleated cells • heterodimer: a chain (43 kD) b2- microglobulin (12 kD) • in humans three different MHC class I molecules: HLA-A, HLA-B and HLA-C • HLA-A, B and C molecules are encoded by separate a chain genes. A single (non-MHC located) gene encodes b2-microglobulin which associates with HLA-A, HLA-B and HLA-C • a1 and a2 domains fold to form b-sheet structure known as peptide binding site (“groove, cleft”) • DNA encoding encoding a1 and a2 domains is very polymorphic (many alleles differ here) • a3 domain/b2-microglobulin fold into “Ig-like” domains

Question 38

MHC class II

Answer 38

• expression limited to APC (and cytokine-activated cells) • heterodimers, a and b chains similar size (34 and 29 kD) and both transmembrane In humans three different MHC class II molecules: HLA-DP, HLA-DQ and HLA-DR • a and b chains encoded by separate genes encoded within MHC • both a2 and b2 domains are Ig-like • similar structure to MHC class I molecules • polymorphic a1 and b1 domains form peptide-binding site • more open than “groove” seen in MHC class I molecules, and binds longer peptides)

Question 39

TCR, CD8 and CD4

Answer 39

``` The TCR expressed by a T cell that co-expresses CD8 can bind MHC class I molecules (that expressed on most cells) The TCR expressed on a T cell that co-expresses CD4 will bind MHC class II molecules expressed on APC ```

Question 40

Summary Antibody

Answer 40

H2L2, expressed on B cells, also secreted Binds free antigens Highly variable, clonally distributed

Question 41

Summary TCR

Answer 41

alpha beta on T cells Binds peptide-MHC complex: dual specificity Highly variable, clonally distributed

Question 42

Summary MHC class I

Answer 42

alpha chain plus beta2-microglobulin, expressed on all nucleated cells Binds peptide 8-10 amino acids to present to TCR Highly polymorphic, limited variation in one individual

Question 43

Summary MHC class II

Answer 43

alpha beta chain heterodimers, expressed on APC Binds peptide 13+ amino acids to present to TCR Highly polymorphic, limited variation in one individual