1. Overview of Ig functions
Shawn Babiuk

2. Overview of Ig Functions
Neutralization
Opsonization
ADCC
Complement
Mucosal Immunity
Immune Evasion
Review through questions and discussion

3. Humoral Immunity Antibodies (secreted proteins produced in B cells)
Extracellular
Mediate several immunological functions
X gammaglobulinemia illustrates the importance of antibodies

4. Roitt, Brostoff, Male Fourth Edition Immunology

5. Antigen recognition
Affinity the strength of the binding between an antibodies binding site and a single epitope
Avidity the functional affinity depends on the number of binding sites on the antibody and their ability to react with multiple epitopes

6. Antibody Isotypes
IgG Neutralization, opsinization, classical complement pathway, ADCC, passive immunity
IgM Pentamer activation of the classical complement pathway
IgA Dimer Mucosal immunity
IgE Mast cell degranulation and parasite killing by eosinophils

7. Toxin neutralization
Binding of antibody to a toxin preventing the toxin from receptors
Examples tetanus toxin
Abbas and Lichtman 2nd addition Basic Immunology

8. Antibody Function Neutralization
Abbas and Lichtman 2nd addition Basic Immunology

9. Neutralization Not all antibodies can neutralize viruses
There is a difference in binding and neutralization
Example: GP120 HIV antibodies can bind but not neutralize HIV
Why? There are certain epitopes that are neutralizing

10. Methods to detect different antibodies functions
ELISA assays detect antibody antigen binding
Virus neutralization assays detect antibodies that prevent viral infection
These two assays can give different results from identical serum samples

11. Opsinization
Antibodies coat microbes and promote their ingestion by phagocytes
Splenectomy leads to increased susceptability to disseminated infections by encapsulated bacteria
Reason: The spleen contains phagocytes and is the major place where phagocytic clearance of opsinized bacteria take place

12. Roitt, Brostoff, Male Fourth Edition Immunology

13. Opsinization
Abbas and Lichtman 2nd addition Basic Immunology

14. Fc Receptors
FcγRI on Macrophages, neutraphils and eosinophils used for phagocytosis and activation of phagocytosis
FcγRIIA on Macrophages, neutraphils and eosinophils and platlets used for phagocytosis
FcγRIIB on B lymphocytes used for feedback inhibition
FcγRIIIA on NK cells used for ADCC
FcεRI on Mast cells, basophils and eosinophils used for cell activation

15. ADCC
Abbas and Lichtman 2nd addition Basic Immunology

16. IgE Mediated killing of parasites
Abbas and Lichtman 2nd addition Basic Immunology

17. Complement A collection of proteins that help in host defence
A sequential sequence of proteolytic cleavage of these proteins lead to the elimination of microbes
Part of the innate immune system, with evolved mechanisms of self/non-self discrimination
Three pathways
Alternative
Classical
Lectin

18. Abbas and Lichtman 2nd addition Basic Immunology

19. Proteins in the early stages of complement (alternative pathway)
C3 -Serum protein in plasma
Low levels of C3 are spontaneously hydrolyzed in plasma to C3b and C3a, where its products are unstable and break down
C3a stimulates inflammation
Factor B broken down to Bb fragment forming C3bBb complex (C3 convertase)
Factor D plasma serine protease that cleaves factor B
Properdin stabilizes the C3bBb convertase
C3bBbC3b is a C5 convertase

20. Proteins in the early stages of complement (classical pathway)
C1(C1q, C1r, C1s) initiates the classical pathway, binds the Fc antibody region protease for C4 and C2
C4 C4b binds to the surface where antibody is bound, C4a stimuates inflammation
C2 binds to C4 where C2a is cleaved by C1 and forms C4b2a (C3 convertase)
C4b2aC3b is a C5 convertase

21. Late stages of complement activation
Abbas and Lichtman 2nd addition Basic Immunology

22. Proteins in late stages of complement activation
C5 C5b initiates assembly of the membrane attack complex (MAC) C5a stimulates inflammation
C6 binds C5b and accepts C7
C7 component of the MAC inserts in to lipid membranes
C8 component of the MAC initiates binding and polymerization of C9
C9 component of the MAC polymerizes to form membrane pores

23. Functions of the Complement System
Important in elimination of microbes during innate and adaptive immune responses
Opsinization
Direct killing of microbes
Chemotactic attraction
Processing of immune complexes
Augmentation of localization of antigen to B cells and APCs

24. Abbas and Lichtman 2nd addition Basic Immunology

25. Roitt, Brostoff, Male Fourth Edition Immunology

26. Inherited deficiencies of complement
C3 deficient susceptible to infections early in life usually fatal
C2 and C4 deficient are normal, indicating that deficiency in the classical and lectin pathways can be overcome by the alternative pathway
C9 MAC deficiency increased Neisseria infections

27. Regulation of Complement
Why? To prevent uncontrolled and harmful activation
Host cells express regulatory membrane proteins
Decay accelerating factor (DAF)
Membrane co-factor protein (MCP)
Type 1 complement receptor (CR1)

28. Complement regulatory proteins
Abbas and Lichtman 2nd addition Basic Immunology

29. Host plasma complement regulatory proteins
C1 Inhibitor inhibits C1r and C1s serine protease activity
Factor I cleaves C3b and C4b
Factor H causes dissociation of alternative pathway C3 convertase subunits co-factor for factor I
C4 binding protein causes dissociation of classical pathway C3 convertase subunits co-factor for factor I

30. Regulation of Complement
Abbas and Lichtman 2nd addition Basic Immunology

31. Complement control
Host cells can be overwhelmed by complement activation
For virus neutralization assays it is therefore important to heat inactivate sera to destroy complement activity

32. Inherited deficiencies of complement regulatory proteins
Deficiency in C1 inhibitor causes hereditary angioneurotic edema a disease characterized by excessive C1 activation causing edema
Deficiency in glycolipid anchor synthesis results in deficiencies in decay accelerating factor and membrane cofactor protein results in uncontrolled complement activation and lysis of erythrocytes

33. Mucosal Immunity
The vast majority of pathogens invade at mucosal surfaces
Blocking pathogens at the site of infection the mucosal surfaces can prevent infection
IgA can be actively transported to and secreted from mucosal surfaces
IgA can be transported from the lamina propria by the poly-Ig receptor on the basal surface of epithelial cells where it is endocytosed and secreted into the lumen where the poly-Ig receptor is cleaved by a protease

34. Neonate immunity
In humans maternal antibodies are actively transported across the placenta (Passive immunity)
In some other animals (cattle) maternal antibodies do not cross the placenta
Antibodies are passively transferred in milk
The presence of maternal antibodies can inhibit immune responses to vaccines

35. Evasion of humoral immunity by microbes
Antigenic variation
Inhibition of complement activation
- Neisseria miningitides sialic acid expression inhibits c3 and c5 convertases
-Streptococcus M protein blocks C3 binding and C3b binding to complement receptors
Staphylococcal protein A binding FC of ABs
Streptocoocal protein G binding FC of ABs
Resistance to phagocytosis Pneumococcus

36. Discussion Questions
What regions of antibodies are involved in the functions of antibodies?

37. Discussion Questions
How do heavy chain class switching and affinity maturation improve the abilities of antibodies to combat infectious pathogens?

38. Discussion Questions
In what situations does the ability of antibodies to neutralize microbes protect the host from infections?

39. Discussion Questions
How is the complement system activated, and why is it effective against microbes but does not react against host cells and tissues?

40. Discussion Questions
What are the functions of the complement system, and what components of complement mediate these functions?

41. Discussion Questions
How do antibodies prevent infections by ingested and inhaled microbes?

42. Discussion Questions
How do neonatal animals develop the capacity to protect themselves from infections even before their immune systems have reached maturity?

43. Discussion Questions
Why are there so many numerous pathways for Ig function?