L3 - Recognition of antigen by B cells and antibody

Question 1

B-cell receptors: what do they do, what do they contain, and what are the parts of the chain called?

Answer 1

Modulates gene expression, adhesion or survival and causes plasma b-cell differentiation

Complementary determining region (CDR/Hypervariable regions)

Two binding parts of the Y shape are Fragment antigen binding (Fab) fragments - also the shape of the TCR, the membrane-bound part is the fragment crystallisation (Fc) site

Question 2

Antibodies: what are they produced by, what are they also called, what do they do, what is their structure, what is the structure of each of its arms, and what domains do they have?

Answer 2

Plasma B cells

IMMUNOGLOBULIN

• Bind foreign antigens encountered by the host
• Mediate effector functions to neutralize or eliminate foreign invaders

Two binding parts of the Y shape are Fragment antigen binding (Fab) fragments - also the shape of the TCR, and the bottom part is the fragment crystallisation (Fc) site

Each arm is a heterodimer of two light chains and two heavy chains (H-L)₂

Vₗ_ - variable domain
Cₗ_ - control domain

Question 3

Papain: what does it do?

Answer 3

Papain cuts at the disulphide bonds that link the heavy chains - two identical fragments with antigen binding activity - Fragment antigen binding or Fab fragment

Question 4

Antigens: what are they, how big are they, and what type of molecules are they?

Answer 4

Molecule recognised by an antibody

Usually big molecules (>4000Da)

• Proteins (most common)
• Carbohydrates (Polysaccharides)
• Lipids
• DNA

Question 5

Antigenic epitopes: what are they, what types are there, and how many potential epitopes are there?

Answer 5

Common, unique, repeating parts of antigens recognised by antibodies

• Linear epitope
• Conformational epitope

The total potential number of antigenic epitopes is enormous: 10¹¹ - 10²⁵

Question 7

Conformational epitopes: how is it formed and how often is it recognised?

Answer 7

Protein folding - may not be a continuous sequence of amino acids

Most antibodies recognise these

Question 8

Linear epitope: how is it formed and how often is it recognised?

Answer 8

Continuous sequence of amino acids

Less common prevalence and therefore recognised less often

Question 9

What forces are involved in antigen-binding?

Answer 9

Non-covalent:
* Hydrogen bonding
* Electrostatic attraction
* Instant dipole
* Van-der-Waals
* Hydrophobic interactions

Question 10

Affinity of antibody binding: what is it, what makes interactions strong and is cross-reactivity possible?

Answer 10

The strength of the binding between an epitope and an antibody-antigen binding site

Interactions are weak unless the two binding molecules are close together

Antibodies may undergo cross-reactivity, binding an epitope that has low affinity due to having bad binding

Question 11

Avidity of antigen binding: what is it and how is avidity strength increased?

Answer 11

overall strength of an antibody-antigen complex

If an antibody is using both binding sites to bind to 2 epitopes on same particle this increases the total binding strength or AVIDITY

Question 12

What is the protective level of antibody

Answer 12

10ng/ml

Question 13

Acquired vs innate immunity: time for action to be taken, specificity, and memory?

Answer 13

Acquired:

• After exposure to a pathogen
• Specific - for a specific pathogen
• Memory - The second response is bigger and faster

Innate immune response:
* Immediate - pre-existing
* Not-specific - for any one pathogen
* No memory - Second response same as the first

Question 14

Effector domain functions

Answer 14

• Neutralisation
• Opsonisation
• Complement activation
• ADCC (Antibody-dependent cell-mediated cytotoxicity)

Question 15

Neutralisation: what is it and what does it do?

Answer 15

Antibodies prevent a pathogen or protein from binding to their target

Important in protection against viruses as this prevents entering the cell and replicating, many vaccines elicit this type of response

Question 16

Opsonisation: what is it, what does it do, and what cell examples of them are there?

Answer 16

Tags pathogens to be phagocytosed

The fc-domain of the antibody bind the Fc-receptor on the phagocyte to trigger phagocytosis

Macrophages, neutrophils, etc

Question 17

Complement activation: what is it and what does it do?

Answer 17

Antibodies attached to the surface of a pathogen activate the first protein in a complement system

This can lead to two things:
* Deposit complement proteins on the surface of the bacterium that leads to pore formation and lyse the bacterium directly
* Complement receptors on phagocytes recognize these complement proteins and induce phagocytosis.

Question 18

Antibody-Dependent Cell-mediated Cytotoxicity (ADCC): what is it and what does it do?

Answer 18

Host cells infected by some viruses can express viral proteins on the surface, these are recognised by antibodies against that particular virus

Cells with FcR (NK cell, macrophage, etc) can bind the antibody and kill the target cell by releasing perforin (lytic enzyme) and granzymes

Mediates extracellular killing (NB: not phagocytosis) ie NK cell activation/attack, can kill Intracellular pathogens

Question 19

Application of antibodies

Answer 19

• Immunotherapy
• Blood-type determination

Question 20

Immunotherapy

Answer 20

Uses immune molecules to attack cancer cells

Question 21

Antibody classes: what are they dependent on, what are the different designed classes to do, are class and antigen-binding specificity 100% linked, what are the different heavy chains called, and what examples are there?

Answer 21

Depends on different C regions used by heavy chains, resulting in class-specific structural and functional properties

• bind to Fc-Receptors
• Complement activation
• Regulation of secretion

No, an antibody can be produced which is a different class but has the same antigen-binding specificity

The different heavy chains that define these classes are called isotypes

igG, igM, igD, igE, and igA

Question 22

igG: how frequent is it, how many classes are there, what does the C region do, what functions do they have, and which form of IgG is most common?

Answer 22

Most abundant

4 classes

C regions contain a cysteine residue essential for polymerization

• Neutralisation
• Opsonisation
• Sensitisation

IgG1 most common

Question 23

IgM: how quickly is it made after Ag contact, what is its form, how many binding sites does it have, and how big is it?

Answer 23

The earliest antibody made after antigen Contact

Pentamer via J chain and disulphide bonds

Up to 10 binding sites

Big - normally in the bloodstream but not tissues, it also increases its avidity for antigens (before affinity maturation)

Question 24

igD: how frequently is it found, where is it found and what does it do?

Answer 24

Rare

Function in serum

Unclear

Question 25

igE: where is it the lowest, what is it produced by, and what does it do?

Answer 25

Lowest in serum

Produced by mast cells

Important in Allergy and worm infection

Question 26

igA: where is it mostly found, how many subclasses does it have, what structure does it have, what does its C regions do, where is it produced, and where does it undergo its function?

Answer 26

Serum, tears, and mucosa (external secretions)

2 subclasses

Is a dimer in secretions – joined by a “J” chain

Contains a cysteine residue essential for polymerization

B cells and antibodies are made in bone marrow and then move to secondary lymphoid tissue - mainly produced in the gut, respiratory epithelium, lactating breast, salivary or tear glands

Serves a very important function at membrane surfaces - the main entry site of pathogens

Question 27

IgA: what are the steps of its transport into the gut?

Answer 27

1 - Basolateral binding of igA, endocytosis, transcytosis to the apical endothelial surface - exocytosis

2 - The poly Ig receptor is found on the basolateral side of intestinal epithelial cells - This attaches to IgA and it is internalised into cell

4 - Part of the poly Ig receptor is cleaved

5 - The remaining part – secretory component (SC) - allows IgA to be transported into the gut

6 - SC helps protect IgA from destruction by gut enzymes

Question 28

Igs and their roles and locations

Answer 28

• Almost all do neutralisation
• IgG is the main opsoniser (some IgA and IgM activity)
• Only IgG does NK-cell killing activation
• IgE mainly does mast cell sensitisation (some igG)
• IgG and IgM are the main c system activators (some IgA)
• Almost all are diffused into extravascular sites
• IgG can transport across the placenta
• IgA (and a lil IgM) transport across the epithelium

Question 29

J chain: what size is it and what does it do?

Answer 29

15kDa allows polymerization by linking to the cysteine of the tail-piece

Question 30

MALT: what is it, what does it do, and how much does it secrete?

Answer 30

Mucosal-associated lymphoid tissue, ie gut - particularly associated with IgA

Major site of B cell activity and antibody production

5g/day!!!!!