1. Antibodies (Immunoglobulins)
Chapter 4

2. Antibody structure and function
Antibodies belong to a family of globular proteins called immunoglobulins.
These proteins can react and bind to antigen. Antibodies are secreted and they also exist as the B-cell receptor (BCR) on the surface of the B-cell.

3. Diversity
Antibodies come in millions of different amino acid sequences and are the most diverse proteins known.
Because the amino acid sequence differs in the arms of various antibody molecules, each different antibody can bind specifically to one unique epitope.
Thus, the arms of an antibody molecule confer the specificity of responses that a host can mount against antigens.
The stem region of an antibody molecule bear its biological activity and defines whether the response against a particular antigen will lead to complement- mediated lysis, enhanced phagocytosis, or (in some cases) allergy. These activities start once antibodies bind to antigen.

4. Biological Activities
The ability to trigger protective physiological activities usually after binding to antigen. This would include:
-Opsonization,
-activation of complement,
-allergic responses,
-neutralize toxins,
-neutralize viruses and other activities

5. Antibody fragments -Digestion With Papain Yields -Pepsin Digestion
2 identical Fab and 1 Fc
Fab Because Fragment That is Antigen Binding
Fc Because Found To Crystallize In Cold Storage
-Pepsin Digestion
F(ab`)2
No Fc Recovery, Digested Entirely
-Mercaptoethanol Reduction (Eliminates Disulfide Bonds) And Alkylation Showed

6. Antibodies classes
The class of antibody is also called the isotype of the antibody. There are 5 isotypes or classes of antibodies in humans: IgM, IgG, IgA, IgE, IgD; these classes are defined by the class of Heavy Chain or H-chain:
IgM - m chain -mu
IgG -g chain -gamma
IgA -a chain- alpha
IgE -e chain -epsilon
IgD - d chain- delta
Differ in structure and in biological activity.

7. IgG Structure

8. IgG Functions Most abundant of all Ig classes
Longest half-life of all the Ig classes
Agglutinates particulate antigens
Precipitates soluble antigens
This is the only antibody class that crosses the placenta
Opsonizes antigen.
IgG that is bound to cells or IgG that is aggregated in antigen-antibody complexes can activate complement.
IgG is good at neutralizing toxin
IgG can interfere with bacterial virulence features such as motility and adherence to tissues
IgG can block virus ability to adhere to target cells
IgG is the main antibody found in the secondary immune response

9. IgG functions continue
Most abundant immunoglobin 80% of serum Ig ~10mg/mL
IgG1,2,3,4 (decreasing serum concentration)
IgG1, IgG3 and IgG4 cross placenta
IgG3 Most effective complement activator
IgG1 and IgG3 High affinity for FcR on phagocytic cells, good for opsonization

10. IgM structure

11. IgM Function IgM monomer is the B-cell receptor of the mature B-cell
IgM is the first antibody made in an immune response, about 7 to 10 days after initial exposure
IgM is the first antibody made in life, about 5 months in utero
IgM is the most primitive antibody class, found even in primitive vertebrates.
IgM is the antibody made to T-independent antigens, polysaccharides
IgM is a very good agglutinating antibody
Natural isohemagglutinins are IgM class
IgM attached to antigen is a very good activator of complement
IgM does not cross the placenta
5-10% of serum immunoglobulin
1.5mg/mL

12. IgA Functions
10-15% of serum IgG
5-15 g of IgA released in secretions!!!!
IgA is the secretory antibody, found in all secretions including colostrum and milk saliva tears mucus.
IgA is particularly important for protection against respiratory and gastrointestinal infectious agents.
IgA is important for passively acquired immunity of nursing baby.
IgA makes lysozyme work better, especially against gram-negative bacteria
IgA neutralizes virus
IgA is transported across membranes via the poly-Ig receptor

13. Draw the structure of IgA

14. IgE functions
Very low serum concentration, 0.3g/mL
IgE is made in response to parasitic worms. When bound to the parasite, it triggers a very strong inflammatory response to the worms.
IgE is the antibody that triggers allergy
IgE binds to Fc receptors on the surface of mast cells and basophils
Binding causes degranulation (Histamine Release)

15. Draw the structure of IgE?

16. Cross-Linkage of Bound IgE Antibody With Allergen Causes

17. IgM and IgD, Expressed on B-cell Surface
We Do Not Know Any Other Biological Effector Activity
Low serum concentrations, ~30g/mL

18. Monoclonal Antibody Applications
Diagnostic Tests
Abs are capable to detect tiny amouns (pg/mL) of molecules
Ex. Pregnancy hormones
Diagnostic Imaging
mAbs that recognize tumor antigens are radiolabeled with iodine I-131
Immunotoxins
mAbs conjugated with toxins
mAbs To Clear Pathogens