General structure of Antibody and its functions ppt
- 1. DR. R. RENUKA, M.SC.,M.PHIL.,PH.D. ASSOCIATE PROFESSOR OF BIOCHEMISTRY V.V.VANNIAPERUMAL COLLEGE FOR WOMEN VIRUDHUNAGAR, TAMILNAD, INDIA.
- 2. ANTIBODY • An antibody, also known as an immunoglobulin (Ig), is a specialized glycoprotein that is produced by plasma cells (activated B cells) after stimulation by an antigen. Antibodies are the functional basis of humoral immunity. Antibodies occur in the blood, in gastric and mucus secretions, and in breast milk. Antibodies in these bodily fluids can bind pathogens and mark them for destruction by phagocytes before they can infect cells.
- 3. GAMMAGLOBULIN Serum treated with OVA Ag Immunized serum The immunoglobulins derive their name from the finding that they migrate with globular proteins when antibody- containing serum is placed in an electrical field.
- 4. FORMS OF Ig • Igs exist in two forms; • Soluble Igs or which are found in dissolved condition in the blood serum and body fluids. • Serum Igs are heterogenous and polyclonal in nature as they are generated against multiple epitopes of the antigens. • Membrane bound Igs which are found on the surface of B cells. They are also called as surface bound Ig or B cell receptors (BCRs). • They play vital role in the initial stages of B cell activation.
- 5. HISTORY
- 6. GENERAL STRUCTURE OF ANTIBODIES • Y- shaped heterodimer, composed of four polypeptide chains. • Two identical short light (L) chains of molecular weight 25,000 Da each and • Two identical long heavy (H) chains of molecular weight 50,000 Da or more. • All the 4 PP chains are linked with each other by disulphide bonds and non-covalent interactions such as hydrogen bonds, hydrophobic bonds, ionic bonds and Vander walls forces. • All chains have 2 ends – an amino terminal end (NH3) and a carboxy terminal end (COOH).
- 7. TYPES OF H AND L CHAIN
- 9. VARIABLE AND CONSTANT REGIONS • Each heavy and light chain in an immunoglobulin molecule contains an amino-terminal variable (V) region that consists of 100 to 110 amino acids and differ from one antibody to another. The remainder of each chain in the molecule – the constant (C) region exhibits limited variation that defines the two light chain subtypes and the five heavy chains subclasses. • The amino terminal portions, corresponding to the variable regions (Fab portion), bind to antigen; effector functions are mediated by the carboxy-terminal constant regions (Fc region).
- 10. VARIABLE REGION • The first 110 amino acid residues near amino terminal end of both L and H chains constitute variable region and are mentioned as VL and VH respectively. • These regions represent Ag binding site or paratope of an antibody. • Some zones (hot spots) within variable region show relatively higher variability in amino acid sequences. These are called as hyper variability region or complementarity determining regions(CDRs). • There are 3 hot spots in L chain and 4 hotspots in H chain. These sites make actual contact with epitope.
- 11. CONSTANT REGION • Remaining part of Ig molecule at the base of the stalk of Y or in the C- terminal end is called constant regions. • Length; 104 amino acids for L chain; 330 AA for 𝜸, 𝜶 𝒂𝒏𝒅 𝜹 heavy chains and 440 AA for 𝝁 𝒂𝒏𝒅 𝜺 heavy chains. • AA sequence of constant region shows uniform pattern. • A single Ab has 2 identical light chains and 2 identical heavy chains.
- 12. DOMAINS • H and L chains further folded into domains due to the presence of intrachain disulphide bonds. • Within a domain, a loop like structure of 60 AA is present. • Each light chain has 2 domains; one VL (variable) domain and the other is CL (constant) domain. • Each heavy chain has four domains; one variable (VH) domain and 3 constant ( CH ) domains which are named as CH1,CH2 and CH3.
- 13. • Ig M and Ig E have an additional constant heavy chain domain called CH4.
- 14. HINGE REGION
- 16. ENZYMATIC DIGESTION OF ANTIBODY REVEALS DIFFERENT FRAGMENTS
- 22. GENERAL FUNCTIONS OF ANTIBODIES • Neutralization of infectivity (Fab fragment), • Phagocytosis, • Antibody-dependent cellular cytotoxicity (ADCC), • Complement-mediated lysis of pathogens or of infected cells: Antibodies activate the complement system to destroy bacterial cells by lysis • Transcytosis, mucosal immunity & neonatal immunity • Another function is unique to Immunoglobulin E (IgE), which is ‘activation of mast cells, eosinophils and basophils’.
- 23. Antibodies are secreted into the blood and mucosa, where they can block the infectivity of pathogens (bacteria, viruses, parasites and fungi), inactivate or neutralize foreign substances such toxins. Neutralization generally occurs as a result of interfering with an organism’s attachment to host tissues.
- 24. Phagocytosis Antibodies facilitate phagocytosis of foreign substances by a process called opsonization. The internalization and degradation of antibody-coated pathogens by macrophages and neutrophils via FcRs (Fc receptors are protein molecules present on the surfaces of macrophages and neutrophils which can bind the constant region of immunoglobulin molecules) is a critical antibody function for clearance of pathogens in vivo.
- 25. Complement-mediated lysis of pathogens or of infected cells • Antibodies (IgM and most IgG subclasses) activate the complement system which can result in the lysis of organisms or of infected cells. An important byproduct of the complement cascade is C3b, which is a protein fragment that can bind non-specifically to cell and Ag-Ab complexes. • Organisms or Ag-Ab complexes bound by complement can be internalized by phagocytic cells, with the resultant clearance. Internalization through complement receptors on antigen-presenting cells (APCs) can also result in the processing of antigen for presentation to T lymphocytes.
- 26. Antibody-Dependent Cellular Cytotoxicity (ADCC) • Antibodies have shown anti-microbial activity either directly or through interactions with FcRs or complement. ADCC occurs when antibody forms a bridge between an infected target cell (virus infected cells of the host) and an FcR-bearing effector cell, particularly natural killer (NK) cells. The result of this three-way interaction is the death of the target cell, either by lysis or apoptosis.
- 27. Transcytosis, Mucosal Immunity and Neonatal Immunity • Some antibodies can move across epithelial layers (depends on the property of the constant region of that antibody molecule) via a process called transcytosis. IgA is the major immunoglobulin that undergoes transcytosis and is available in secretory form (sIgA) in the mucosal surfaces of respiratory, gastrointestinal and urogenital tracts. • In mammalian species including humans, most subclasses of IgG can cross the placental barrier which confers protection of developing foetus against pathogens. This passive immunization of developing foetus occurs during the third trimester of gestation.
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