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Transcript
Immunoglobulin Structure and Function By Associate Lecturer Mortadha H AL-Hussainy Faculty of Veterinary Medicine Kufa University Immunogobulin, Ig • What is Immunoglobulin? Immunoglobulin are the critical ingredients of humoral acquired immune response. • The immunoglobulins are a group of glycoproteins present in the serum and tissue fluids of all mammals. Immunoglobulins:Structure and Function • Definition: Glycoprotein molecules that are produced by plasma cells in response to an immunogen and which function as antibodies Amount of protein + albumin globulins α1 α2 β γ Immune serum Ag adsorbed serum Mobility General Functions of Immunoglobulins • Ag binding – Can result in protection – Valence • Effector functions (Usually require Ag binding) – Fixation of complement – Binding to mast cells , macrophages, NK cell Basic Immunoglobulin Structure • Immunoglobulins - heterogeneous • Myeloma proteins - homogeneous immunoglobulins Two Forms of Immunoglobulin Membrane-bound receptor Soluble antibody Immunoglobulin Structure Disulfide bond • Variable(V) & Constant (C) Regions – VL & CL – VH & CH • Hinge Region Carbohydrate CL VL CH2 CH1 VH Hinge Region CH3 Structural Regions • hypervariable region • also called Complementarity Determining Regions(CDRs), 超变区( hyper-variable region, HVR),又称互补决定区 (complementary determining region, CDR) IgG molecule Used with permission from: Dr. Mike Clark, Immunology Division, Department of Pathology Cambridge University, Cambridge, England Enzymatic Digestion Products of Immunoglobulins Immunoglobulin Fragments: Structure/Function Relationships • Fab Papain – Ag binding – Valence = 1 – Specificity determined by VH and VL • Fc ( crystallizable) – Effector functions Fc Fab Domains of Immunoglobulin Functions of the domains on Ig: VH, VL — antigen binding sites; CH1~3, CL — genetic markers of Ig; CH2(IgG), CH3(IgM) — C1q binding sites; CH2~CH3(IgG) — binding to placenta; CH3(IgG) — FcγR binding site; CH4(IgE) — FcεR binding site. Function of Immunoglobulins • Recognition of antigen 识别抗原 • Activation of complement 激活补体 • Opsonization 调理作用 • Antibody-dependent cell-mediated cytotoxicity,ADCC 抗体依赖性细胞毒作用 • Mediate hypersensitivity type I 超敏反应 Immunoglobulin Classes and Subclasses Immunglobulin molecules are divided into distinct classes and subclasses in terms of the differences in amino acid sequence of constant region of heavy chain, i.e.γ,α,μ,δ,andεchains. Immunoglobulin Classes of Mammals • • • • • IgG - Gamma (γ) heavy chains IgM - Mu (µ) heavy chains IgA - Alpha (α) heavy chains IgD - Delta (δ) heavy chains IgE - Epsilon (ε) heavy chains Five Classes of Immunoglobulin • IgG has a family of subclass, IgG1, IgG2, IgG3, IgG4(cattle has no) • IgA is divided into two subclasses, IgA1 and IgA2(sheep). Light Chain Types of Immunoglobulin • Kappa (κ) • Lambda (λ) • All light chains have protein molecular weights of approximately 23,000 but can be divided into two distinct types, namely λchain, κchain, respectively B Cell Antigen Receptor (BCR) Ig-α Ig-β Ig-α Ig-β IgA • Structure – Serum - monomer – Secretions (sIgA) • Dimer (11S) • J chain • Secretory component Secretory Piece J Chain IgA • Structure • Properties – 2nd highest serum Ig – Major secretory Ig (Mucosal or Local Immunity) • Tears, saliva, gastric and pulmonary secretions – Does not fix complement (unless aggregated) – Binds to Fc receptors on some cells IgD • Structure • Properties – 4th highest serum Ig – B cell surface Ig – Does not bind complement IgE • Structure • Properties – Least common serum Ig • Binds to basophils and mast cells (Does not require Ag binding) – Allergic reactions – Parasitic infections (Helminths) • Binds to Fc receptor on eosinophils – Does not fix complement