4. Antibodies Structure and Function 2024.pdf

Mohammad Altamimi, MD, PhD Hashemite University Faculty of Medicine Immunology 2024 Objectives  Immhuglobuline structure and binding site/s  Immunoglobiuline classes and their characteristics  the role of Immuoglobulines in neutralization, opsonization, antibody-depen...

Mohammad Altamimi, MD, PhD Hashemite University Faculty of Medicine Immunology 2024 Objectives  Immhuglobuline structure and binding site/s  Immunoglobiuline classes and their characteristics  the role of Immuoglobulines in neutralization, opsonization, antibody-dependent cellular cytotoxicity (ADCC), complement and mucosal immunity  Introduction to artificial antibodies including monoclonal and polyclonal antibodies Introduction  Antibodies are proteins that recognize and bind to a particular antigen with very high specificity.  Belong to a group of serum proteins called immunoglobulins (Igs).  Ab is produced by activated B cells (plasma cells) in response to a stimulation of Ag.  Ab possesses a high degree of specificity and affinity  Each antibody has at least two identical sites that bind antigen: Antigen binding sites.  Each Ig contains three hypervariable regions, or complementary determining region (CDRs) Antibodies Structure  Immunoglobulins are glycoproteins made up of Four polypeptide chains (IgG):  Two light (L) polypeptide chains  Two heavy (H) polypeptide chains  The four chains are linked by disulfide bonds  The four chains are assembled to form a Y-shaped molecule  Each domain folds into a characteristic three-dimensional shape, called the immunoglobulin (Ig) domain  Ig domains without hypervariable loops are present in many other proteins in the immune system, as well as outside the immune system  All of these proteins are said to be members of the “immunoglobulin superfamily” Variable (V) and Constant (C) Regions  Each H-chain and each L-chain has V-region and C-region 1. V- region: The terminal portion of the L-chain and terminal portion of the H-chain compose the antigen binding site and are located within the “Fab” (fragment, antigen-binding) region. It shows wide variation in amino acid sequences 2. C-region: lies in the carboxyl or terminal portion of the molecule. C-region shows an unvarying amino acid sequence and forms the “Fc” (fragment crystalline) region. It is responsible for biological functions. H-chains are distinct for each of the five Immunoglobulins  There are five types of Ig heavy chains, called μ, δ, γ, ε, and α, which differ in their C regions ((IgM, IgD, IgG, IgE, and IgA)  Two types of light chain, termed lambda (λ) and kappa (κ)  An antibody molecule is composed of two identical Ig heavy chains (H) called μ, δ, γ, ε, and α and two identical light chains (L) termed lambda (λ) and kappa (κ), each with a variable region (V) & constant region (C). Antibodies Classes  Five classes of Antibodies: 1. IgG 2. IgM 3. IgA 4. IgD 5. IgE 1. IgG  Structure: Monomer  Percentage serum antibodies: 80%  Location: Blood, lymph, intestine  Half-life in serum: 23 days  Complement Fixation: Yes  Placental Transfer: Yes  Known Functions: Enhances phagocytosis, neutralizes toxins and viruses, protects fetus and newborn. Four subclasses: IgG1, IgG2, IgG3, IgG4 2. IgM  Structure: Pentamer  Percentage serum antibodies: 5-10%  Location: Blood, lymph, B cell surface (monomer)  Half-life in serum: 5 days  Complement Fixation: Yes  Placental Transfer: No  Known Functions: First antibodies produced during an infection. Effective against microbes and agglutinating antigens. 3. IgA  Structure: Dimer  Percentage serum antibodies: 10-15%  Location: Secretions (tears, saliva, intestine, milk), blood and lymph.  Half-life in serum: 6 days  Complement Fixation: No  Placental Transfer: No  Two subclasses：IgA1，IgA2  Known Functions: Localized protection of mucosal surfaces. Provides immunity to infant digestive tract. 4. IgD  Structure: Monomer  Percentage serum antibodies: 0.2%  Location: B-cell surface, blood, and lymph  Half-life in serum: 3 days  Complement Fixation: No  Placental Transfer: No  Known Functions: In serum function is unknown. On B cell surface, initiate immune response. 5. IgE  Structure: Monomer  Percentage serum antibodies: 0.002%  Location: Bound to mast cells and basophils throughout body. Blood.  Half-life in serum: 2 days  Complement Fixation: No  Placental Transfer: No  Known Functions: Allergic reactions. Possibly lysis of worms. Antibodies Functions 1. Neutralization: Bind antigen- neutralize toxins, virus particles 2. Opsonization 3. Complement activation- IgG, IgM 4. Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC) 5. Mast cells activation 6. Transcytosis- movement across epithelial cells 1. Neutralization  The first step in a microbial infection involves attachment of the organism to the outside surface of the human body, either some part of the skin or the mucosal surfaces  High-affinity antibodies that bind to the microbial ligand and prevent the microbe’s attachment to human epithelium stop the infection before it starts  Antibodies thus bind and inactivate foreign antigenic entities directly. 2. Opsonization  Many bacteria are coated with polysaccharide → slippery and hard to endocytose  But IgG can bind polysaccharide  Macrophage can specifically bind IgG via FC-γ receptors 3. Compliment Activation  Classical: IgM or 2 adjacent IgG’s binds to C1Q on bacterial surface results in cascade that can cause bacterial lysis  Alternative: antibody binding attracts C3B → phagocytosis and opsonization 4. Mast Cell Activation  IgE exists in serum at very low concentration (ng/ml)  IgE binds to FC-ε recptors on Mast, Basophil, and Langerhan cells  Antigen cross links bound antibodies → degranualtion and release of histamine, heparin, proteases, chemotaxins which attracts WBC’s  This induce Phospholipase activity → mucus production, sneezing and other allergic symptoms 5. ADCC: Antibody-Dependent Cell Mediated Cytotoxicity  IgG binds target cell (virally infected or tumorigenic)  FC-γR on NK (non B, non T, natural killers) bind IgG  Crosslinking of receptors → perforin/protease release by NK 6. Transport/trancytosis  Submucosal lymphoid follicles secretes IgA (trachea, for example)  Epithelial M cells phagocytose/pinocytose foreign particles in lumen transport digested antigens into follicle and stimulate antibody production  Placental transport: active transport of IgG across placenta give protection for baby up to six months Artificial Antibodies  Antibodies made artificially  Tow types: 1. Polyclonal Ab:  A mixture Ab with different specificities and affinities  Generate in a natural response or artificial immunization 2. Monoclonal Ab:  Ab produced by single clone (or one hybridomas clone) and having a single specificity Monoclonal Ab Applications  Diagnostic Tests  mAbs 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  mAbs for treatment (thrombotic diseases, cancer..)  Artificial antibodies POLYCLONAL. MONOCLONAL. Derived from different B Derived from a single B cell Lymphocytes cell lines clone mAb offer Reproducible, Batch to Batch variation Predictable & Potentially affecting Ab reactivity & inexhaustible supply of Ab titre with exquisite specificity NOT Powerful tools for Enable the development of clinical diagnostic tests secure immunoassay systems.