Immunoglobulin are proteins of animal origin endowed with or without antibody activity.
So structurally related proteins without antibody activity are also known as immunoglobulin, e.g. myeloma protein (found in multiple myeloma).
Antibodies are glycoprotein substances synthesized and secreted by B lymphoid lineage cells, termed plasma cells, in response to stimulation with an immunogen.
Basic structure of Immunoglobulin (Ig)
• Shape of immunoglobulin – “Y” shape structure. Its upper end is called amino terminal & lower end is called carboxyl terminal.
• Consists of 4 polypeptide chains. 1 pair of heavy (H) chain & 1 pair of light (L) chain. Both chains are held together by a number of disulphide bonds. Every Ig consists of equal number of light & heavy chain polypeptides.
• The H & L chains are both composed of folded globular domains, each of which is 100 – 110 amino acid long & contains a single intrachain disulphide bond. The L chain contains 2 of these domains & H chain contains either 4 or 5 of these domains.
• Molecular weight of light chains are 25,000 Da. There are 2 light chain types: kappa (κ) & lambda (λ). 60% of light chains are kappa & 40 % are lambda. A normal antibody molecule contains only one light chain type, either κ or λ, never both.
• Molecular wt. of heavy chains are 50,000 – 70,000 Da. There are 5 types of heavy chain: α, δ, ε, μ, γ. The heavy chains of an antibody molecule determine the class of that antibody: IgM (μ), IgG (γ), IgA (α), IgE (ε),
• Amino acid sequence of amino terminal of both heavy & light vary greatly. This region is called variable (V) region. Rest of the portion of chains remain relatively constant & are called constant (C) region.
• L chain contains 1 variable (VL ) & 1 constant (CL ) region. H chain contains 1 variable (VH) & 3 (IgG, IgD & IgA) or 4 (IgM & IgE) constant regions (CH ).
• Within the variable regions of both L & H chains there are 3 extremely variable portions called hyper variable (CDRs) regions, which contain highly variable amino acid sequence. This form Ag binding site of Ab.
• Hinge region: the γ, δ, α heavy chains contain an extended peptide sequence between CH1 & CH2 domains is called hinge region. This area is flexible to enable the antibody to bind to pairs of epitopes at various distances on an antigen. Although μ & ε chains lack hinge region, they have an additional domains of 110 amino acids (CH2/CH2) that has hingelike features.
• If an antibody is treated by papain, the peptide bonds at hinge region are broken, produce 2 identical fragments known as Fab (fragment antigen binding) & a third fragment known as Fc (fragment crystallisable).
Biological activity of Fc portion:
1. Activate complement pathway (IgG & IgM).
2. Bind to phagocytes (IgG).
3. Bind to basophil, mast cell & eosinophil (IgE).
4. Bind to NK cells (IgG)
Classes & subclasses of immunoglobulin
5 classes of Ig:
• IgG – IgG1, IgG2, IgG3, IgG4.
• IgA – IgA1, IgA2.
Immunoglobulin G (IgG)
• IgG is present in serum at a concentration of 12 mg/ml & constitutes about 80% of total serum Ig.
• Structure: consists of 2 γ heavy chains & 2 κ or 2 λ light chains.
• Distribution: equally between extravascular & intravascular compartment.
• Mol. Wt. – 1,50,000 Da
• Subclasses: 4 subclasses IgG1, IgG2, IgG3, IgG4. Distinguished by differences in γ chain constant region.
• Biological functions:
1. It is the main antibody in secondary immune response.
2. It is the only antibody (IgG1, IgG3, IgG4) that can cross placenta (due to presence of receptor for Fc portion of IgG in syncytiotrophoblast).
3. It opsonize bacteria & enhance their phagocytosis.
4. It can agglutinate particular antigens & precipitate soluble antigens.
5. Activate classical pathway of complement (IgG1, IgG2, IgG3).
6. Plays important role in ADCC.
7. Neutralize bacterial toxins & viruses.
Immunoglobulin M (IgM)
• Concentration in serum: 1.5mg/ml. 5 – 10% of total serum Ig.
• Distribution: predominantly in the intravascular space.
• Molecular wt. : 9,00,000 Da
• Structure: it is present as monomer on the surface of B cell. IgM is secreted by plasma cells as a pentamer in which 5 mononmeric units are held together by additional disulphide bonds between Fc fragments. In addition, there is a joining polypeptide chain, known as J (joining) chain.
• Biological function:
1. Agglutinate antigen.
2. Activate complement.
3. Neutralize toxins
4. Has antiviral activity.
Immunoglobulin A (IgA)
• Serum concentration: 10 – 15% of total serum Ig.
• Distribution: predominant Ig in external secretions such as breast milk, saliva, tears & mucus of bronchial, genitourinary & GIT.
• Structure: in serum, it exists as monomer. Polymeric forms (dimers, trimers & some tetramers) are present in external secretions, called secretory IgA. Secretory IgA also contains J chain & a polypeptide chain called secretory component, produced by epithelial cells of mucus membrane. The secretory component protects IgA from being degraded in the intestinal tract.
• Subclasses: IgA1 & IgA2 . IgA1 predominates in serum (80%), IgA2 percentages are higher in secretions than in serum (35% in secretions).
• Biological functions:
1. Provides primary immunologic defense against local infections of GIT, respiratory tract, eye etc.
2. Agglutinate antigen.
3. Activate alternative pathway of complement in aggregated form.
Immunoglobulin E (IgE)
• Also known as reaginic antibody.
• Serum concentration: 0.3 μg/ml.
• Mol. Wt. : 2,00,000 Da.
1. It mediates immediate hypersensitivity reaction.
2. Provides protection against helminthes.
Immunoglobulin D (IgD)
• Found on the surface of B cells as well as in serum.
• Serum concentration: 30 μg/ml & 0.2% of total serum Ig.
• Mol. Wt. : 1,80,000 Da.
• Biologic functions:
1. Play role in eliminating B cells generating self reactive antibody.
Differences between 5 Immunoglobulin(Ig) classes:
How antibody defend the body?
2. Cytolysis by MAC: mediated by IgG, IgM & aggregated IgA.
3. ADCC by NK cells:
4. Neutralization of exotoxin: antibody binds to toxin prevent its binding to receptor & neutralize toxin. Ab-toxin complex then cleared by phagocytic cells.
5. Neutralization of virus: Ab neutralize virus by –
- Binding to specific host cell receptor for particular virus.
- Blocking entry of virus by binding to epitopes necessary for fusion of virus with host cells
- Blocking uncoating of virus.
6. Agglutination of microorganism.
7. Immobilization of bacteria & protozoa.
8. Mucosal immunity by secretory IgA.
Antigenic determinants of Immunoglobulin
• Ig molecules can themselves, when injected into other animal species, functions as potent immunogens to induce an antibody response.
• These antigenic determinants fall into 3 major categories:
1. Isotype: isotypic determinants are defined by antigenic differences in their constant region of both heavy & light chains.
2. Allotype: within a species multiple alleles exist for some of the genes. These alleles encode subtle amino acid differences, called allotypic determinants. The sum of individual allotypic determinants displayed by an antibody determine its allotype. In human, allotypes have been characterized for all 4 IgG subclasses, 1 IgA subclass & for κ light chain.
3. Idiotype: The idiotypic determinants arise from the sequence of the heavy & light chain variable regions. Each antigenic determinants of the variable region is referred to as an idiotope. The sum of the individual idiotopes is called the idiotype of the antibody.
• As a B cell clone proliferates, individual daughter cells often appear that express a heavy chain class (such as γ or α) that differs from that of the founder.
This phenomenon is called immunoglobulin class switching or isotype switching or isotypic commutation or class switch recombination (CSR).
• During this process, the constant-region portion of the antibody heavy chain is changed, but the variable region of the heavy chain stays the same. Since the variable region does not change, class switching does not affect antigen specificity. Instead, the antibody retains affinity for the same antigens, but can interact with different effectors molecules.
• Class switching depends on interplay of 4 elements:
- Switch regions
- Switch recombinase
- Activation induced cytidine deaminase.
• In expressed heavy chain gene a new CH region is moved to a position adjacent to the existing V/D/J exon by deleting all intervening CH sequences on the chromosome.
• Class switching is irreversible because switching occur here by deleting one or more heavy chain isotype gene.
Antibody Class Switch Recombination (CSR)
Cytokines regulate Class switching
• Monoclonal antibodies are monospecific antibodies that are the same because they are made by identical immune cells that are all clones of a unique parent cell.
• Eg. Infliximab, Adalimumab,
• Use of monoclonal antibody:
1. As diagnostic reagent –
- ✓ Detect pregnancy.
- ✓ Diagnosis of pathogenic organism.
- ✓ Measuring blood level of various drugs.
- ✓ Detecting antigens shed by certain tumor.
- ✓ CRP
- ✓ ASO.
2. Radiology & imaging: Monoclonal antibody to tumor specific antigen related to breast cancer is labeled with iodine-131 is introduced into blood to detect the speed of tumor to regional lymph node.
3. As a therapy-
- ✓ Monoclonal antibody tagged with toxin. It is called immunetoxin.
- ✓ Viral disease
- ✓ Anticancer therapy
- ✓ Autoimmune disease- Rheumatoid arthritis, SLE, Leukaemia, Hodgkins disease, Chrons disease.