TODAY

1. TODAY ANTIBODY STRUCTURE AND FUNCTION

2. Antigens are detected by specialized T cell receptors (TCR) or B cell receptors (BCR). APC T cell B cell T cells require antigen presentation by dedicated antigen presenting cells (APC) The BCR directly recongnizes its cognate antigen

3. Antigen recognition is mediated by structurally related cell surface molecules: membrane-bound antibodies on B lymphocytes and T cell receptors on T lymphocytes. MICROBE B CELL Antigen receptors distinguish many, and often chemically related, structures. Antigen receptors are clonally distributed: Each clone has a unique receptor distinct from all other receptors Clone: A parental lymphocyte and all its progeny. LYMPHOCYTE ACTIVATION IS TRIGGERED BY ANTIGEN RECOGNITION

4. BCR/TCR ACTIVATION BCR COMPLEX 1. Antigen receptors are non-covalently associated with cellular signaling proteins in the BCR or TCR complex. Extracellular BCR 2. When adjacent antigen receptors bind two or more antigens the receptors are pulled together into an aggregate. Intracellular 3. Cross-linking brings signaling proteins together and initiates signal transduction. 4. As the signaling proteins are the same in each clone, the transduced signal is also the same.

5. B CELLS 1. B CELLS ARISE FROM BONE MARROW STEM CELLS. 2. MATURE B CELLS EXIT BONE MARROW AND ENTER THE CIRCULATORY SYSTEM. 3. B CELLS RECIRCULATE THROUGH LYMPHOID ORGANS IN SEARCH OF ANTIGEN.

6. B CELLS 1. BCR detect three-dimensional antigens. 2. B cell activation does not require antigen presentation. 3. B cells are active only against extracellular antigens.

7. B CELLS AND ANTIBODIES 1. Each B cell makes One specificity of antibody (Immunoglobulin (Ig)). The antigen receptor is membrane bound on a naïve B cell. 2. Ag plus second signal stimulate B cell. 3. Active B cell secretes antibody into the blood and lymph or patrolling mucosal surfaces.

8. B CELLS AND ANTIBODIES Secreted antibodies BCR Naive Active In the transition from naïve to active B cells, the B cells secrete a modified version of the B cell receptor. This molecule is known as an antibody. The BCR and corresponding antibody share IDENTICAL antigen specificities.

9. What are Antibodies? Antibodies (A.K.A. Immunoglobulin) are a type of glycoprotein produced by B lymphocytes. Antibodies bind antigen with a high degree of specificity and affinity. Antibodies recognize a variety of three-dimensional shapes (amino acids, lipids, carbohydrates, etc).

10. An antibody consists of four polypeptides: two identical light chains and two identical heavy chains form a Y-shaped molecule. Each light chain is connected to a heavy chain by a disulfide bond and the two heavy chains are connected by two disulfide bonds. LIGHT CHAIN Antigen binding and effector domains are separated by a hinge region. The hinge region allows the two antigen binding domains to move, enabling them to bind antigens that are separated by varying distances. HEAVY CHAIN ANTIBODY STRUCTURE

11. 1. The variable (V) region varies between clones and is involved in antigen recognition. 2. The constant (C) region is conserved among clones and is required for structural integrity and effector functions. ANTIBODY STRUCTURE ANTIGEN RECEPTORS HAVE DISTINCT FUNCTIONAL DOMAINS

12. Antigen IMMUNITY IMMUNITY IMMUNITY ANTIBODY STRUCTURE The unique structural architecture of antibodies allows multiple, highly diverse antigens to induce identical effector functions. Variable region Constant region Different variable regions bind different antigens. Identical constant regions induce identical responses.

13. Each Light chain contains one V domain and one C domain. Each Heavy chain contains one V domain and at least three C domains. Each domain folds into a characteristic 3-D shape: The Immunoglobulin (Ig) domain. VL Antibodies bind antigens through the variable regions of the light and heavy chains. VH ANTIBODY STRUCTURE

14. Epitope Epitope Epitope: The parts of an antigen recognized by an antibody are called epitopes. Epitopes can be recognized on the basis of sequence or shape. Affinity: The strength with which one antigen-binding surface of an antibody binds an antigen. Avidity: Each Isotype has between 2 and 10 antigen binding sites. Therefore, each antibody can bind 2 to 10 epitopes of an antigen, as long as identical epitopes are sufficiently close together, e.g. microbial cell surface proteins. In this case the binding is much greater than the affinity of a single antibody-antigen bond and is called the avidity. BINDING OF ANTIGEN BY ANTIBODY - TERMS AND CONCEPTS Antibodies bind antigens by reversible non-covalent interactions.

15. Specific Antibody (Ig) Molecules Are Tailored to Exactly Fit A 3 D Surface Site On An Antigen. Epitope Influenza coat protein Ig

16. 1. Antibodies recognize a large array of 3-D structures. 2. Each clone is specific for a single antigen. 3. Antigen recognition is mediated by specific domains of the antibody. 4. Signaling triggers B lymphocyte activation. FEATURES OF ANITBODY-MEDIATED ANTIGEN RECOGNITION

17. B CELL ACTIVATION 1. Antigen 1. Antigen 2. PAMP 2. PAMP PRR PRR BCR BCR Active B cell B cell Cytokines 1. Ag CD4+ T cell BCR MHC II/Ag B cell B cell activation requires two signals A. T Cell-independent. 1st signal: BCR detects Ag. 2nd signal: PRR detects PAMP. B. T Cell-dependent. 1st signal: BCR detects Ag. B cell internalizes Ag and presents to CD4+ T cell (B cell as APC). 2nd signal: T cell produces cytokines that activate B cell.

18. IMMUNOGLOBULIN ISOTYPES IgA. Two IgA molecules joined by J chain. The J chain facilitates transport of IgA across mucosal epithelia. The J chain also facilitates transfer of IgA to newborns to confer neonatal passive immunity. There are five types of heavy chain that differ in their C region. Each class is an Isotype. Each Isotype differs in their physical/biological properties and effector functions.

19. IMMUNOGLOBULIN ISOTYPES IgD. Function unknown. Restricted to membrane and not expressed on active B lymphocytes. IgD knock-out mice do not have any apparent defects.

20. IMMUNOGLOBULIN ISOTYPES Antigen IgE Fc Receptor Secreted as a monomer. MAST CELL IgE binds the Fc receptors of mast cells. When IgE is cross-linked by antigen, it triggers mast cell degranulation - allergic response.

21. IMMUNOGLOBULIN ISOTYPES Secreted as monomer IgG can bind and neutralize toxins.

22. IMMUNOGLOBULIN ISOTYPES Opsonization: IgG coats pathogens and prevents them from entering host cells. Infectious pathogen entering host cell to propagate Pathogen cannot enter cell when coated (“opsonized”) with antibody

23. IMMUNOGLOBULIN ISOTYPES Ag Phagolysosme IgG FcR Macrophage MHC II/peptide 1. IgG-Ag binds the Fc receptor of macrophages. 2. Ag is internalized through phagocytosis. 3. Microbe is degraded in the phagolysosome. 4. Microbial peptides are presented on Class II MHC molecules - CD4 T cell activation!

24. IMMUNOGLOBULIN ISOTYPES Natural Killer Cells Target Ig Ag Fc receptors also mediate Antibody-Dependent Cellular Cytotoxicity (ADCC) 1. Ig binds Ag on surface of target cell. 2. Fc Receptors on Natural Killer cells bind Fc of Ig. 3. Cross-linking of Fc receptors signals to the NK cell to kill the target. 4. Target cell dies by apoptosis

25. IMMUNOGLOBULIN ISOTYPES IgM exists as pentamers 10 different Ag binding sites - very high avidity! As with IgA, it has a J chain for secretion. IgM is the first antibody expressed in mature B cells. Important for activation of the complement pathway. Appears early after infection and usually not seen upon re-infection.