TODAY ANTIBODY STRUCTURE AND FUNCTION Asilmi 06 B

TODAY ANTIBODY STRUCTURE AND FUNCTION Asilmi 06 - B CELLS AND ANTIBODIES

Antigens are detected by specialized T cell receptors (TCR) or B cell receptors (BCR). APC T cells require antigen presentation by dedicated antigen presenting cells (APC) B cell The BCR directly recongnizes its cognate antigen Asilmi 06 - B CELLS AND ANTIBODIES

LYMPHOCYTE ACTIVATION IS TRIGGERED BY ANTIGEN RECOGNITION Antigen recognition is mediated by structurally related cell surface molecules: membranebound 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. Asilmi 06 - B CELLS AND ANTIBODIES

2. When adjacent antigen receptors bind two or more antigens the receptors are pulled together into an aggregate. Extracellular BCR Intracellular 1. Antigen receptors are noncovalently associated with cellular signaling proteins in the BCR or TCR complex. BCR COMPLEX BCR/TCR ACTIVATION 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. Asilmi 06 - B CELLS AND ANTIBODIES

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. Asilmi 06 - B CELLS AND ANTIBODIES

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. Asilmi 06 - B CELLS AND ANTIBODIES

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. Asilmi 06 - B CELLS AND ANTIBODIES

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. Asilmi 06 - B CELLS AND ANTIBODIES

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). Asilmi 06 - B CELLS AND ANTIBODIES

ANTIBODY STRUCTURE 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. 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. LIGHT CHAIN HEAVY CHAIN Asilmi 06 - B CELLS AND ANTIBODIES

ANTIBODY STRUCTURE ANTIGEN RECEPTORS HAVE DISTINCT FUNCTIONAL DOMAINS 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. Asilmi 06 - B CELLS AND ANTIBODIES

ANTIBODY STRUCTURE The unique structural architecture of antibodies allows multiple, highly diverse antigens to induce identical effector functions. Antigen Variable region Constant region IMMUNITY Different variable regions bind different antigens. Identical constant regions induce identical responses. Asilmi 06 - B CELLS AND ANTIBODIES

ANTIBODY STRUCTURE 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 Asilmi 06 - B CELLS AND ANTIBODIES

BINDING OF ANTIGEN BY ANTIBODY - TERMS AND CONCEPTS Epitope Antibodies bind antigens by reversible noncovalent interactions. 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. Asilmi 06 - B CELLS AND ANTIBODIES

Specific Antibody (Ig) Molecules Are Tailored to Exactly Fit A 3 D Surface Site On An Antigen. Epitope Influenza coat protein Ig Asilmi 06 - B CELLS AND ANTIBODIES

FEATURES OF ANITBODY-MEDIATED ANTIGEN RECOGNITION 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. Asilmi 06 - B CELLS AND ANTIBODIES

B CELL ACTIVATION B cell activation requires two signals 1. Antigen A. T Cell-independent. 1 st signal: BCR detects Ag. 1. Antigen 2. PAMP PRR BCR 2 nd signal: PRR detects PAMP. B cell B. T Cell-dependent. 1 st signal: BCR detects Ag. B cell internalizes Ag and presents to CD 4+ T cell (B cell as APC). 2 nd signal: T cell produces cytokines that activate B cell. 1. Ag Active B cell Cytokines CD 4+ T cell BCR MHC II/Ag B cell Asilmi 06 - B CELLS AND ANTIBODIES

IMMUNOGLOBULIN ISOTYPES 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. Ig. A. Two Ig. A molecules joined by J chain. The J chain facilitates transport of Ig. A across mucosal epithelia. The J chain also facilitates transfer of Ig. A to newborns to confer neonatal passive immunity. Asilmi 06 - B CELLS AND ANTIBODIES

IMMUNOGLOBULIN ISOTYPES Ig. D. Function unknown. Restricted to membrane and not expressed on active B lymphocytes. Ig. D knock-out mice do not have any apparent defects. Asilmi 06 - B CELLS AND ANTIBODIES

IMMUNOGLOBULIN ISOTYPES Antigen Ig. E Secreted as a monomer. Fc Receptor MAST CELL Ig. E binds the Fc receptors of mast cells. When Ig. E is cross-linked by antigen, it triggers mast cell degranulation - allergic response. Asilmi 06 - B CELLS AND ANTIBODIES

IMMUNOGLOBULIN ISOTYPES Secreted as monomer Ig. G can bind and neutralize toxins. Asilmi 06 - B CELLS AND ANTIBODIES

IMMUNOGLOBULIN ISOTYPES Opsonization: Ig. G 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 Asilmi 06 - B CELLS AND ANTIBODIES

Ag IMMUNOGLOBULIN ISOTYPES Phagolysosme Ig. G Fc. R Macrophage MHC II/peptide 1. Ig. G-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 - CD 4 T cell activation! Asilmi 06 - B CELLS AND ANTIBODIES

IMMUNOGLOBULIN ISOTYPES Fc receptors also mediate Antibody-Dependent Cellular Cytotoxicity (ADCC) Natural Killer Cells Ig Target Ag 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 Asilmi 06 - B CELLS AND ANTIBODIES

IMMUNOGLOBULIN ISOTYPES Ig. M exists as pentamers 10 different Ag binding sites - very high avidity! As with Ig. A, it has a J chain for secretion. Ig. M 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. Asilmi 06 - B CELLS AND ANTIBODIES