L3 - Recognition of antigen by B cells and antibody Flashcards
B-cell receptors: what do they do, what do they contain, and what are the parts of the chain called?
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
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?
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
Papain: what does it do?
Papain cuts at the disulphide bonds that link the heavy chains - two identical fragments with antigen binding activity - Fragment antigen binding or Fab fragment
Antigens: what are they, how big are they, and what type of molecules are they?
Molecule recognised by an antibody
Usually big molecules (>4000Da)
- Proteins (most common)
- Carbohydrates (Polysaccharides)
- Lipids
- DNA
Antigenic epitopes: what are they, what types are there, and how many potential epitopes are there?
Common, unique, repeating parts of antigens recognised by antibodies
- Linear epitope
- Conformational epitope
The total potential number of antigenic epitopes is enormous: 10¹¹ - 10²⁵
Conformational epitopes: how is it formed and how often is it recognised?
Protein folding - may not be a continuous sequence of amino acids
Most antibodies recognise these
Linear epitope: how is it formed and how often is it recognised?
Continuous sequence of amino acids
Less common prevalence and therefore recognised less often
What forces are involved in antigen-binding?
Non-covalent:
* Hydrogen bonding
* Electrostatic attraction
* Instant dipole
* Van-der-Waals
* Hydrophobic interactions
Affinity of antibody binding: what is it, what makes interactions strong and is cross-reactivity possible?
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
Avidity of antigen binding: what is it and how is avidity strength increased?
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
What is the protective level of antibody
10ng/ml
Acquired vs innate immunity: time for action to be taken, specificity, and memory?
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
Effector domain functions
- Neutralisation
- Opsonisation
- Complement activation
- ADCC (Antibody-dependent cell-mediated cytotoxicity)
Neutralisation: what is it and what does it do?
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
Opsonisation: what is it, what does it do, and what cell examples of them are there?
Tags pathogens to be phagocytosed
The fc-domain of the antibody bind the Fc-receptor on the phagocyte to trigger phagocytosis
Macrophages, neutrophils, etc
Complement activation: what is it and what does it do?
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.
Antibody-Dependent Cell-mediated Cytotoxicity (ADCC): what is it and what does it do?
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
Application of antibodies
- Immunotherapy
- Blood-type determination
Immunotherapy
Uses immune molecules to attack cancer cells
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?
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
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?
Most abundant
4 classes
C regions contain a cysteine residue essential for polymerization
- Neutralisation
- Opsonisation
- Sensitisation
IgG1 most common
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?
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)
igD: how frequently is it found, where is it found and what does it do?
Rare
Function in serum
Unclear
igE: where is it the lowest, what is it produced by, and what does it do?
Lowest in serum
Produced by mast cells
Important in Allergy and worm infection
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?
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
IgA: what are the steps of its transport into the gut?
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
Igs and their roles and locations
- 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
J chain: what size is it and what does it do?
15kDa allows polymerization by linking to the cysteine of the tail-piece
MALT: what is it, what does it do, and how much does it secrete?
Mucosal-associated lymphoid tissue, ie gut - particularly associated with IgA
Major site of B cell activity and antibody production
5g/day!!!!!
