Antibody

antibody are produced by the plasma cells in the body against any type of antigen

1. ANTIBODY Mrs. Praveen Garg VITS College, Satna

2. I ntroduction An antibody, also known as an immunoglobulin. It is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of the pathogen, called an antigen . Antibodies  are specialized protective proteins that bind like a lock-and-key to the body's foreign invaders , whether they are viruses, bacteria, fungi or parasites . When an antigen binds to the B-cell surface, it stimulates the B cell to divide and mature into a group of identical cells called a clone. The mature B cells, called plasma cells, secrete millions of antibodies into the bloodstream and lymphatic system.

3. A ntibody structure Antibody is heterodimers structrure , made up of two pair of polypeptide chain The basic unit is composed of two identical light (L) chains and two identical heavy (H) chains, which are held together by disulfide bonds to form a flexible Y shape. Each chain is composed of a variable (V) region and a constant (C) region .

4. Variable (V) and constant (C) domains within the light (L) and heavy (H) chains of an antibody, or immunoglobulin, molecule. The folded shapes of the domains are maintained by disulfide bonds (―S―S―).

5. Diversity in the variable domain Variable region contain amino terminal end and constant region contain carboxyl terminal end. There are two light chain: kappa (k) and lambda chain ( λ ). Amino acid sequence of a large number of V L and V H domains revealed that more sequence variation occur in variable region. These region mainly called hypervariable region. It form the antigen binding site of the antibody molecule. Because the antigen binding site is complementary to the structure of epitope , these area are now called complementarity determining regions (CDRs). CDR is the antigen binding sites

6. Types of antibody Antibodies are grouped into five classes according to their constant region and in activity. Each class is designated by a letter attached to an abbreviation of the word  immunoglobulin :  IgG ( γ ),   IgM ( μ ) ,   IgA ( α ),   IgD ( δ ) , and  IgE ( ε ) IgG , the most common antibody, is present mostly in the blood and tissue fluids. IgA is found in the mucous membranes lining the respiratory and gastrointestinal tracts as secreted form. IgM  found in blood and lymph fluid and are the first type of antibody made in response to an infection.  IgE antibodies are found mostly in the lungs, skin, and mucous membranes and small found in blood. This can lead to an allergic reaction. They cause mast cells to release chemicals, including histamine, into the bloodstream .   IgD  is also produced in a secreted form that is found in small amounts in blood serum, nasal and mucous secretion.

7. IgG Immunoglobulin  G ( IgG ) antibodies are large globular proteins with a molecular weight of about 150 kDa made of four peptide chains. It contains two identical γ (gamma) heavy chains of about 50 kDa and two identical light chains of about 25 kDa , thus a tetrameric quaternary  structure . Immunoglobulin G ( IgG ), the most abundant type of antibody, is found in all body fluids and protects against bacterial and viral infections. It represents approximately 75% of serum antibodies in humans and thus the most common type of antibody found in the circulation. Each end of the fork contains an identical antigen binding site. Thus, each IgG has two antigen binding sites. The heavy chain components are CH1, CH2, CH3, hinge and the VH (variable region of heavy chain) and light chains consist of CL (Constant region of light chain) and the κ or λ chains. The Fc regions of IgGs bear a highly conserved N- glycosylation site.

8. There are four IgG subclasses (IgG1, 2, 3, and 4) in humans, named in order of their abundance in serum (IgG1 being the most abundant). Subclasses IgG1, IgG2, IgG3, and IgG4 are differentiated on the basis of the size of the hinge region, position of interchain disulfide bonds, and molecular weight.  The subclasses also differ in their ability to activate complement. IgG Subclasses

9. IgG1 It occurs 60 to 65% of the total IgG , and responsible for the thymus-mediated immune response against proteins and polypeptide antigens and involved in opsonization and activation of the complement proteins. A deficiency in IgG1 isotype is a sign of a hypogammaglobulinemia . IgG2 It occurs 20 to 25% of the total IgG and generate immune response against carbohydrate/polysaccharide antigens. A deficiency in IgG2 is the most common and is associated with respiratory infections in infants. IgG3 IgG3 occurs 5 to 10% of total IgG and plays a major role in the immune responses against protein or polypeptide antigens. IgG4 It occurs less than 4% of total IgG , IgG4 does not bind to polysaccharides. Elevated serum levels of IgG4 are found in patients suffering from sclerosing pancreatitis, cholangitis (inflammation in bile duct) and interstitial pneumonia caused by infiltrating IgG4 positive plasma cells.

10. Function of IgG IgG is the main type of antibody found in blood and extracellular fluid and control infection of body tissues. IgG is the only class of immunoglobulin that can cross the placenta in humans, and it is largely responsible for protection of the newborn during the first months of life. IgG is the major immunoglobulin in blood, lymph fluid, cerebrospinal fluid and peritoneal fluid and a key player in the humoral immune response. The binding of the Fc portion of IgG to the receptor present on a phagocyte is a critical step in the opsonization . IgG is produced in a delayed response to an infection and can be retained in the body for a long time. The longevity in serum makes IgG most useful for passive immunization by transfer of this antibody. Detection of IgG usually indicates a prior infection or vaccination. Because of its relative abundance and excellent specificity toward antigens, IgG is the principle antibody used in immunological research and clinical diagnostics.

11. IgA The basic monomer unit of  IgA , in common with all antibodies, is arranged into two identical Fab regions, linked through the hinge region to the Fc region, which mediates effector mechanisms. Immunoglobulin A ( IgA ) is the first line of defence in the resistance against infection, via inhibiting bacterial and viral adhesion to epithelial cells and by neutralisation of bacterial toxins and virus, both extra and intracellularly . Immunoglobulin A ( IgA ), is the major class of antibody present in the mucosal secretions of most mammals. It is present in mucous, tears, saliva, sweat,  colostrum  and secretions from the genitourinary tract, gastrointestinal tract, prostate and respiratory epithelium. It is also found in small amounts in blood.  It occurs15% of the total serum immunoglobulin and thus is the second most common human immunoglobulin in serum with serum concentration of 1 to 4 mg/ml.

12. IgA subclasses Two IgA subtypes exist in humans, IgA1 und IgA2. They differ in the molecular mass of the heavy chains and in their concentration in serum. IgA1 Predominant circulating IgA1 comprising 85% of total IgA concentration in serum. IgA1 shows a good immune response to protein antigens and, to a less response against polysaccharides and lipopolysaccharides . IgA2 IgA2, represents only up to 15% of total IgA in serum, but IgA2 percentages are higher in secretions. Plays a crucial role in the mucosa of the airways, eyes and the gastrointestinal tract to fight against polysaccharide and lipo-polysaccaride antigens.

13. Structure of IgA Immunoglobulin A ( IgA ) is present in the serum as a 170 kDa , four polypeptide (two L and two H) chain protein. Its H-chain type is alpha (α). IgA exists in serum in both monomeric and dimeric forms. Although it exists primarily in monomeric form, followed by dimeric , trimeric and some tetrameric forms are also present. IgA in blood occurs in monomeric form whereas those in body secretion occur in dimeric or multimeric forms. In secretions, in addition to the κ or λ L-chains and the IgA heavy chain alpha, IgA also contains two other polypeptide chains – secretory component (SC) and J-chain (Joining chain). Secretory chains help in transcytosis of exocrine IgA and stabilize IgA against proteolytic degradation. The two four-chain units are held together by the J-chain through disulfide bridges.

14. Functions of IgA It primarily protects mucous membranes as IgA can cross the epithelial layer and enter into body secretion providing local immunity. Secretory IgA provides the primary defense mechanism against some local infections because of its abundance in mucosal secretions (e.g., saliva and tears). It prevents the passage of foreign substances into the circulatory system. In body secretion IgA neutralize viruses and prevent attachment on host surface. Secretory IgA can also inhibit inflammatory effects of other immunoglobulins .

15. IgE IgE exists as a monomer and is the least abundant antibody isotype in plasma, present in a circulation and accounts for 0.002% of total immunoglobulin. IgE has a very short half-life which is less than 1 day. The reason is because some proportion of the circulating amount is continually removed and destroyed in endosomes . Despite the low concentrations in the circulation, IgE is extremely biologically active. IgE antibodies bind to high-affinity receptors present on the surface of mast cells and basophils , so that these cells may be highly sensitive to allergens and release histamine chemicals. Because of this high-affinity interaction, almost all IgE produced by B cells is bound to mast cells or basophils , which also explains the low concentration found in circulation. Clinical diseases associated with unusually high elevation of serum IgE concentrations include atopic diseases (asthma, allergic rhinitis, atopic dermatitis, urticaria ), parasitic diseases, cutaneous diseases, neoplastic diseases, and immune deficiencies.

16. Structure of IgE IgE is a glycoprotein of 190KDa molecular weight produced as a monomeric antibody comprising of 2 heavy and 2 light chains linked by disulfide bonds. IgE is found similar to IgG in structure, but IgE comprises two unique features: The heavy chain which consists of high (12%) carbohydrate content and an additional constant region (CH4). The structure has two identical antigen binding areas consisting of both light chain (23KDa) and heavy chains (70KDa). The light chains have two domains, one constant and one variable. The heavy chains have five domains, one variable and four constant -region domains. It is unique in having the additional constant region (CH4). The unique CH4 region restricts IgE binding to high-affinity receptors ( Fce -RI) on the surface of basophils and mast cells, which contain preformed granules of heparin and histamine. A hinge region is absent in the structure of IgE .

17. IgM IgM is an antigen receptor on B cells and the first antibody produced in an immune response. It is present both on B cells, and as a soluble molecule in the blood. Because of its large size (900 kDa ), IgM is found in the intravascular space i.e. in the bloodstream and also lymph fluid. Serum  IgM exists as a pentamer and comprises 10% of normal human serum Ig content. It is the third most abundant human immunoglobulin. It produce first in primary immune responses to most antigens and is the most efficient complement-fixing immunoglobulin.

18. IgM

19. A pentamer , where all heavy chains and all light chains are identical. A monomer found on B lymphocytes as B cell receptors. On the B cell surface this molecule is expressed on the plasma membrane as a monomer. It contain an additional hydrophobic domain for anchoring in the membrane. In the blood, IgM is composed of pentamer unit held together by disulfide bridges at the carboxy – terminal end of the H chains. Each of the five monomers within the pentamer structure is composed of two light chains (either kappa or lambda) and two heavy chains. The heavy chain in IgM monomers is composed of one variable and four constant regions. A hinge region is also absent in this antibody . J-chain is also associated with IgM in the blood and initiates the polymerization of its subunits at the time of its secretion from a plasma cell. Structure of IgM

20. IgM is the first antibody produced as primary immune response. It is responsible for agglutination, neutralizing and cytolytic reactions and is of vital importance in complement activation. IgM interacts with several other physiological molecules such as bind with complement  component  C1 , binds to the  polyimmunoglobulin receptor ( pIgR )  in a process that brings IgM to mucosal surfaces, such as the gut lumen and into breast milk etc and have role in mucosal defence . IgM is the first immunoglobulin class to be synthesized by the neonate and plays a role in the pathogenesis of some autoimmune diseases. Although IgM antibodies usually have low-affinity binding sites for antigen, they have ten combining sites per molecule which can synergize with each other. It thus has high avidity for antigens and is very efficient per molecule in dealing with pathogens especially early in the immune response. Elevated levels of IgM can be a sign of recent infection or exposure to antigen. function of IgM

21. IgD IgD is also produced in a secreted form that is found in very small amounts in blood serum, representing 0.25% of immunoglobulins in serum. IgD has a molecular mass of 185  kDa  while a half-life of 2-3 days, similar to that of IgE . It is co-expressed with another cell surface antibody,  IgM . B cells thus can express both IgM and IgD which are specific for the same antigen. IgD starts to be expressed when the B cell exits the bone marrow to populate peripheral lymphoid tissues. When a B cell reaches its mature state, it co-expresses both  IgM  and IgD . IgD is also produced in a secreted form that is found in very small amounts in blood serum.

22. Structure of IgD Secreted IgD is a glycoprotein produced as a  monomeric  antibody with two identical heavy chains and two identical light chains. The light chains have two domains, one variable and one constant. The heavy chains have four domains, one variable and three constant region domains.  The IgD molecule has a long “hinge” region (between Fab and Fc ) that appears the molecule very susceptible to proteolytic degradation with production of Fab and Fc fragments and also renders the molecule flexible, thus enhancing antigen binding.

23. Secreted IgD also exists and plays an exclusive function in blood, mucosal secretions and on the surface of innate immune effector cells such as basophils . IgD is also able to bind to basophils and mast cells and activate these cells to produce antimicrobial factors that are functional in respiratory immune defence in humans.  It is also proposed that IgD may have some role in allergic reactions. Function of IgD

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