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Antibodies belong to a class of antigen binding proteins called immunoglobulins. Antibodies are present on the B-cell membrane and secreted by plasma cells . Tag particles for clearance/destruction . Protect against re-infection ( vaccines ).

Serum and plasma:

Serum and plasma Blood Blood + anticoagulant Cells Plasma Serum Clot

Basic Structure of Antibodies:

Basic Structure of Antibodies Electrophoresis of one serum aliquot revealed four peaks corresponding to albumin and the alpha ( α ) , beta ( β ) , and gamma ( ϒ ) globulins. Treated with ovalbumin (To absorb antibody)

Antibody Structure:

Antibody Structure

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Antibodies are made up of: 2 Light Chains (identical) ~25 KDa 2 Heavy Chains (identical) ~50 KDa Each light chain bound to heavy chain by disulfide (H-L). Heavy Chain Bound to Heavy Chain (H-H). First 100 a/a of amino terminal vary of both H and L Chain are variable referred to as V L , V H , C H And C L CDR (Complementarity Determining Regions) are what bind Ag . Remaining regions are very similar within same class.

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Repeating Domains of ~110 a/a Intrachain disulfide bonds within each domain Heavy chains 1 V H and either 3 or 4 C H (C H 1, C H 2, C H 3, C H 4) Light chains 1 V L and 1 C L Hinge Region Rich in proline residues ( flexible ) Hinge found in IgG , IgA and IgD Proline residues are target for proteolytic digestion ( papain and pepsin) Rich in cysteine residues (disulfide bonds) IgM and IgE lack hinge region They instead have extra C H 4 Domain

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Digestion With Papain Yields 3 Fragments ( 2 identical Fab and 1 Fc ) Fab Because Fragment That is Antigen Binding Fc Because Found To Crystallize In Cold Storage Digestion With Pepsin Yields F( ab `)2 No Fc Recovery, Digested Entirely Mercaptoethanol Reduction (Eliminates Disulfide Bonds) And Alkylation Showed Enzymatic Digestion of Antibodies

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Enzymatic Digestion of Antibodies

Antibody Classes and Biological Activities:

Antibody Classes and Biological Activities The structures of the five major classes are: Immunoglobulin G ( IgG ) Immunoglobulin M ( IgM ) Immunoglobulin A ( IgA ) Immunoglobulin E ( IgE ) Immunoglobulin D ( IgD )

Immunoglobulin G (IgG):

Immunoglobulin G ( IgG ) Constitutes about 80 % of the total serum immunoglobulin . The IgG molecule consists of two ϒ heavy chains and two κ or two λ light chains . There are four human IgG subclasses IgG1, IgG2 , IgG3 , and IgG4

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The subtle amino acid differences between subclasses of IgG affect the biological activity of the molecule IgG1, IgG3 , and IgG4 readily cross the placenta protect the developing fetus. IgG3 is the most effective complement activator . IgG1 and IgG3 bind with high affinity to Fc receptors on phagocytic cells and thus mediate opsonization. IgG4 has an intermediate affinity for Fc receptors, and IgG2 has an extremely low affinity

Immunoglobulin M (Ig M):

Immunoglobulin M ( Ig M ) IgM accounts for 5%–10% of the total serum immunoglobulin . IgM is secreted by plasma cells as a pentamer in which five monomer units are held together by disulfide bonds that link their carboxyl- terminal heavy chain domains (C4/C4) and their C3/C3 domains. The five monomer subunits are arranged with their Fc regions in the center of the pentamer and the ten antigen-binding sites on the periphery of the molecule . Each pentamer contains an additional Fc -linked polypeptide called the J ( joining) chain . The J chain appears to be required for polymerization of the monomers to form pentameric IgM

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Because of its large size , IgM does not diffuse well and therefore is found in very low concentrations in the intercellular tissue fluids. IgM is the first immunoglobulin class produced in a primary response to an antigen . it is the first immunoglobulin to be synthesized by the neonate.

Immunoglobulin A (IgA):

Immunoglobulin A ( IgA ) IgA constitutes only 10%–15% of the total immunoglobulin in serum . IgA is secreted in external secretions such as breast milk , saliva, tears, and mucus of the bronchial, genitourinary, and digestive tracts . IgA exists primarily as a monomer , but polymeric forms ( dimers , trimers , and some tetramers ) are sometimes seen. Secretory IgA serves an important effector function at mucous membrane surfaces , which are the main entry sites for most pathogenic organisms. Secretory IgA provide an important line of defense against bacteria such as Salmonella, Vibrio cholerae , and Neisseria gonorrhoeae and viruses such as polio, influenza , and reovirus .

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Breast milk contains secretory IgA and many other molecules that help protect the newborn against infection during the first month of life. Because the immune system of infants is not fully functional, breast-feeding plays an important role in maintaining the health of newborns.

Immune benefits of breast milk:

Immune benefits of breast milk

Immunoglobulin E (Ig E):

Immunoglobulin E ( Ig E ) Its extremely low average serum concentration 0.3 g/ml . IgE binds to Fc receptors on basophils and mast cells ; induces hypersensitivity reactions that are responsible for the symptoms of hay fever, asthma, hives, and anaphylactic shock .

Immunoglobulin D (IgD):

Immunoglobulin D ( IgD ) Constitutes about 0.2% of the total immunoglobulin in serum . No biological effector function has been identified for IgD .

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