Antibody, Immunoglobulin

 

                                               ANTIBODY                 By Anup Bajracharya

An antibody is a specialized protein produced by the immune system, specifically by B cells, in response to the presence of foreign substances called antigens. Antibodies are glycoprotein molecules called immunoglobulins. They react specifically with the antigen that induced their production.

Antibodies role:

• Recognizing and binding to foreign substances and facilitating their removal.
• Increasing phagocytosis.
• Neutralizing toxins or viruses.
• Activating complement.

Basic Structure

An antibody molecule has a basic structure (roughly Y-shaped) composed of four polypeptide chains connected by disulfide bonds. These four chains include two identical heavy (H) chains (the longer chains) and two identical light (L) chains (the shorter ones).Each chain contains a variable region (N-terminus) and a constant region (C-terminus).The four chains are assembled to form a Y-shaped molecule. Each light chain is attached to one heavy chain, and the two are attached to each other, all by disulfide bonds, to form a monomeric unit.

 

 

• Light chains:
• An immunoglobulin monomer contains two identical κ (kappa) or two identical λ (lambda) light chains, but never one of each.
• Each light chain usually consists of about 220 amino acids and has a molecular weight of 25000 daltons.
• They contain a variable (VL) domain and a constant (CL) domain.
• Each domain contains about 110 amino acids and an intrachain disulfide bond.
• The amino acid sequence in the VL domain is different between immunoglobulins synthesized by different B cells. The CL domain's sequence is constant.
• Heavy chains:
• There are five types of heavy chains: mu (µ), delta (ð), gamma (γ), epsilon (ϵ), and alpha (α).
• Each heavy chain consists of about 440 amino acids and has a molecular weight of about 50-70 kilodalton.
• Heavy chains contain one variable (VH) domain and three or four constant (CH) domains.
• The ð, γ, and α heavy chains contain three constant domains (CH1, CH2, CH3).
• The µ and ϵ heavy chains contain a fourth constant domain (CH4), making them both longer and heavier than γ, ð, or α heavy chains.

• When the antibody is cleaved at the hinge region with the papain enzyme, it gives two Fab fragments and one Fc fragment.

 

 Antigen-Binding Sites (Fab)

• The amino portion (NH terminus) of a single heavy and a single light chain form an epitope-binding site.
• A light chain variable domain (VL) and a heavy chain variable domain (VH) together form a cavity that constitutes the antigen (epitope)-binding region.
• Because an immunoglobulin monomer contains two identical light chains and two identical heavy chains, the two binding sites found in each monomeric immunoglobulin are also identical.

Fc Region

• The constant regions of the heavy chains for all molecules of the same immunoglobulin class have virtually the same amino acid sequence.
• This constant region (Fc region) is attached to the cell surface like Macrophages, PMNs, Monocytes, etc
• When the antibody is cleaved with papain enzyme, it gives one Fc fragment.

Immunoglobulin Isotypes/Classes

• IgG:
• Makes up the greatest amount of immunoglobulin in the serum (80% - 85%).
• Is the major immunoglobulin in extravascular spaces.
• Structure: Monomer with two γ heavy chains and two κ or λ light chains. Fc region has CH1, CH2, and CH3 domains.
• Is a 7S immunoglobulin with a molecular weight of 160 KDa.
• There are four human IgG subclasses: IgG1 (65-70%), IgG2 (23-28%), IgG3 (4-7 %), and IgG4 (3-4%).
• Many IgG antibodies effectively activate complement, opsonizing and neutralize microorganisms and viruses, and initiate antibody-dependent cell-mediated cytotoxicity (ADCC).
• Is the only class of antibodies that can cross the placenta from mother to fetus, mediated by a receptor on placental cells for the Fc region of IgG. Not all subclasses cross equally well; IgG2 does not cross well.
• Fixes complement, though not all subclasses fix equally well; IgG4 does not fix complement.
• Macrophages, monocytes, PMNs, and some lymphocytes have Fc receptors for the Fc region of IgG.
• Produced later than IgM but provides long-lasting immunity.

 

 

• IgM:
• Found either as a cell surface-bound monomer (on unstimulated B cells) or as a secreted pentamer (5%-10 % of serum Ig).
• Is generally the first immunoglobulin to be formed following antigenic stimulation.
• Structure: Pentamer (secreted) or monomer (B cell surface).
• Pentamer consists of five monomers linked by disulfide bonds between heavy chains and a J Chain. All heavy chains in the pentamer are identical, as are all light chains. Each monomer has two μ heavy chains and two κ or λ light chains.
• 
  Secreted IgM is a 19S immunoglobulin. Monomeric IgM is 180KDa. IgM pentamer is more than five times larger than IgG.

• Each heavy chain has an additional domain (CH4) at the C terminus of the Fc region.
• IgM does not have a hinge region.
• The valence is theoretically 10 (in the pentameric form).
• Predominantly present intravascularly
• Is the first immunoglobulin made by the fetus or a virgin B cell upon stimulation.
• Is a good complement fixing immunoglobulin due to its pentameric structure.
• Is a good agglutinating immunoglobulin (immobilizing antigen), very efficient in clumping microorganisms.
• The multiple antigen-binding sites make it very efficient in functions like phagocytosis.
• Binds to some cells via Fc receptors.

 

• IgA:

• Present in both monomeric (in serum, 10% - 13% of serum Ig) and dimeric forms (in secretions).
• Serum IgA is a monomer (7S, 160 KDa).
• Secretory IgA (dimer) is found in mucosal surfaces and secretions.
• Dimer is formed by two IgA monomers joined by a J chain and also with a secretory component (SC).
• Epithelial cells transport the IgA dimer to mucosal surfaces using a specialized receptor, which becomes the SC.
• Secretory IgA dimers (11S, 360KDa) are found in mucus, saliva, tears, breast milk, and gastrointestinal secretions.
• IgA molecule has two α heavy chains containing three constant domains (CH1, CH2, CH3) and either two κ or two λ light chains.
• Important in local (mucosal) immunity.
• Normally does not fix complement, unless aggregated.
• Can bind to some cells (PMN's and some lymphocytes).
• Helps in phagocytosis and intracellular killing of microorganisms.
• Minor component in systemic humoral immunity, major role in mucosal immunity.
• IgA antibodies found in gut contents or feces are known as copro antibodies.

 

 

• IgD:
• (1% of serum Ig) has a monomeric structure. Found in low levels in serum; its role in serum is uncertain.
• Almost exclusively displayed on B-cell surfaces, where it functions as a receptor for antigen.
• IgD on B cells has extra amino acids at the C-terminal end for anchoring to the membrane.
• Does not bind complement.

 

 

• IgE:
• Present in relatively low serum concentration.
• Most is adsorbed on the surfaces of mast cells and eosinophils.
• Mast cells and basophils have specific receptors for the Fc portion of free IgE molecules.
• Mediates immediate hypersensitivity reactions (Type-1), such as anaphylactic shock, hay fever, asthma.
• Cross-linking of IgE on mast cell surfaces by antigen triggers the release of histamine and other inflammatory mediators.
• Involved in allergic reactions.
• Plays a role in parasitic helminth diseases. Measuring IgE levels helps diagnose parasitic infections.
• Does not fix complement.