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ImmunoglobulinStructure and Function
By
Associate Lecturer Mortadha H AL-Hussainy
Faculty of Veterinary Medicine
Kufa University
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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.
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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
Am
oun
t of
pro
tein
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General Functions of Immunoglobulins
• Effector functions – Fixation of complement– Binding to mast cells , macrophages, NK cell
(Usually require Ag binding)
• Ag binding– Can result in protection– Valence
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Basic Immunoglobulin Structure
• Immunoglobulins - heterogeneous
• Myeloma proteins - homogeneous immunoglobulins
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Two Forms of Immunoglobulin
Membrane-bound receptor Soluble antibody
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Immunoglobulin Structure
• Variable(V) & Constant (C) Regions– VL & CL
– VH & CH
• Hinge Region
CH1
VL
CL
VH
CH2 CH3
Hinge Region
Carbohydrate
Disulfide bond
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Structural Regions
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• hypervariable region
• also called
Complementarity Determining Regions(CDRs),
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超变区超变区(( hyper-variable region, HVRhyper-variable region, HVR)),, 又称又称互补决互补决定定区区 (complementary determining region, CD(complementary determining region, CDRR ))
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IgG molecule
Used with permission from: Dr. Mike Clark, Immunology Division, Department of Pathology Cambridge University, Cambridge, England
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Enzymatic Digestion Products of Immunoglobulins
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Immunoglobulin Fragments: Structure/Function Relationships
• Fab– Ag binding– Valence = 1– Specificity
determined by VH and VL
Papain
Fc
Fab
• Fc ( crystallizable)– Effector functions
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Domains of Immunoglobulin
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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.
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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
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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.
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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
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Five Classes of Immunoglobulin
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• IgG has a family of subclass, IgG1, IgG2, IgG3,
IgG4(cattle has no)
• IgA is divided into two subclasses, IgA1 and
IgA2(sheep).
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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
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B Cell Antigen Receptor (BCR)
Ig-αIg-β Ig-βIg-α
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IgA
• Structure– Serum - monomer– Secretions (sIgA)
• Dimer (11S)• J chain• Secretory component
J ChainSecretory Piece
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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
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IgD
• Structure
• Properties– 4th highest serum Ig– B cell surface Ig– Does not bind complement
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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