Lecture 8: Humoural Immunity

Question 1

The Ig-fold

Answer 1

• 12.5kD B-barrel ~ 100 amino acids
• 2 anti-parallel B-pleated sheets
• Sheets joined by central disulphide bond
• B-strands joined by 3 loops

Question 2

Fab fragment components

Answer 2

6 loops, 2H and 2L chains joined by inter-chain disulphide

Question 3

6 important antibody functions

Answer 3

• Oponsisation- antibody combines with complement to coat targets for phagocytosis
• Block adherence- stops bacteria adhering to mucosal cells for entry. IgA is best for this
• Neutrolisation- direct binding to toxins to block receptor binding. E.g anti-tetanus
• Agglutination- clumping cells together as a result of multivalent anitbody. Used diagnostically
• Immobilisation - binds to bacterial flagellum to inhibit movement
• Antibody-mediated cell cytotoxicity - antibodies bind target on cell and triggers natural killer cell via Fc receptor binding. Best shown with IgG and IGE.

Obnoxious bitches never actually ingest cum

Question 4

IgM: General information

Answer 4

• Default isotype made by all B cells
• Membrane bound and soluble forms
• The B-cell receptor
• 5 chain joined by J chain
• Up to 10 binding sites for antigen
• Low affinity, high avidity
• Reacts to surfaces
• Powerful opsonin
• Fixes complement

Question 5

IgG: General Information

Answer 5

• Major serum class
• Placental immunity - only subclass
• Anti-toxin (soluble)
• Anti-bacterial (surface)
• Opsonises
• Complement fixer
• Blocks receptor binding
• High affinity and high specificity
• 4 subclasses in humans (G1,G2, G3,G4)

Question 6

IgA: General Information

Answer 6

• Serum & mucosal surfaces: e.g guts, lungs, saliva & breast milk
• First line of defence at mucosa
• Blocks pathogen adhesion
• Secreted form linked by J-chain and secretory component (protects and transports across epithelia)
• Dimer

Question 7

IgE: General Information

Answer 7

• Produced by plasma B cells in mucosa (lungs, guts)
• Defence against large organisms (e.g parasites) complex antigens (e.g pollen)
• Potent activator of mast cells
• High affinity FcER receptor on mast cells - may already be armed with IgE
• • Potent activator of mast cells
• • Type 1 hypersensitivity
• • Atopic allergy
• • Allergy and anaphylaxis

Question 8

IgD: General Information

Answer 8

• Structurally similar to IgG
• Evolutionarily old conserved
• Dual IgM, IgD expressio (same specificity) on B cells prior to class switching
• IgD + B cells can resist central tolerance
• Secreted IgD has a role in maintaining mucosal homeostasis - arming mast cells and basophils

Question 9

Fc receptor

Answer 9

• Each Ig class has its own unique Fc receptor
• FcR are expressed on myeloid cells, activation can lead to:
• • macrophage phagocytosis
• • release of mediators and cytotoxins from effector cells including mast cells, basophils, eosinophils, and granulocytes
• Fc receptors are generally low affinity and only bind Ab-Ag complexes inducing receptor cross-linking
• IgE FcER is the exception. High affinity for monomeric IgE. Causes atopic allergy

Question 10

Complementarity Determining Regions

Answer 10

• Amino acid variability confined to complementarity determining regions in the VH and VL domains
• CDRs are the 3 loops in the VH and VL domains that join the strands making 6 loops altogether

Question 11

Molecular Forces: AbAg binding

Answer 11

Hydrogen bonding
Electrostatic
Van der Waals
Hydrophobic