Immunoglobulin — Structure and Function 孟 明教授, PhD，MD

Immunogobulin, Ig What is Immunoglobulin? Immunoglobulins 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 Immune serum Ag adsorbed serum α1 α2 β γ + - albumin globulins Mobility Amount of protein

General Functions of Immunoglobulins Ag binding Can result in protection Valence Effector functions Fixation of complement Binding to mast cells , macrophages, NK cell (Usually require Ag binding)

Basic Immunoglobulin Structure Immunoglobulins - heterogeneous Myeloma proteins - homogeneous immunoglobulins

Two Forms of Immunoglobulin Membrane-bound receptor Soluble antibody

Immunoglobulin Structure Variable(V) & Constant (C) Regions VL & CL VH & CH Hinge Region CH1 VL CL VH CH2 CH3 Hinge Region Carbohydrate Disulfide bond

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 Ag binding Valence = 1 Specificity determined by VH and VL Papain Fc Fab Fc ( crystallizable) Effector functions

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)

IgA Structure Serum - monomer Secretions (sIgA) Dimer (11S) J chain Secretory component J Chain Secretory Piece

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 Allergic reactions 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

To be continued

Isotypes, Allotypes, and Idiotypes of Immunoglobulins

Isotype 同种型 All the heavy chain constant region structures which give rise to classes and subclasses are expressed together in the serum of a normal subject are called Isotypic variants. Each of those is called Isotype.

Allotypes 同种异型 Amino acid sequences of immunoglobulins differ from each other in a same species, and this is called inherited sequence variation. The variations in heavy chain genes are called Allotypes.

Idiotypes 独特型 The variations of amino acid sequences within the variation domains on light and heavy chains are called Idiotypes.

Physiology of Immunoglobulins 1. IgG(in chicken IgY): opsonization, complement activation, antibody-dependent cell-mediated cytotoxicity (ADCC). 2. IgA(chicken): mucosal immunity. 3. IgD: naive B cell antigen receptor. 4. IgM(chicken): naive B cell receptor, complement activation. 5. IgE: immediate hypersensitivity.

Transport Across Placenta

Antibody-Dependent Cell-Mediated Cytotoxicity(ADCC)

Phagocytosis of antibody-antigen Neutralization Bacterial toxins Host cell Toxin receptors Neutralization by antibody Phagocytosis of antibody-antigen complex by macrophage Fc receptor Forming phagosome

Opsonization Extracellular bacteria Opsonization Macrophage Ingestion by macrophage Digestion in lysosome

Complement Activation Digestion in lysosome Bacteria in plasma Complement activation Lysis and ingestion

Fate of Antibody-Toxin or Antibody-Pathogen Complexes Lysosome Phagosome fuses with lysosome, antigen–antibody complex is digested by lysosomal hydrolases Phagosome

Antibody Technology Polyclonal antibody Monoclonal antibody Engineer-based antibody single chain antibody

scFv (single chain V region antibody)

Fv (Variable region fragment)

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