Antibodies and their types with functions

structure , types, functions

1. Antibody Structure, Types and Functions B.ASHOK KUMAR ASSISTANT PROFESSOR KRK GOVT DEGREE COLLEGE ADDANKI-523201 [email protected]

2. Antibody Immunoglobulin 20% of total plasma proteins-Humoral immunity These are Glycoproteins belong to Immunoglobulin’s super family They constitute most of the gamma globulins of blood proteins. Rodney Porter and Gerald Edleman revealed the structure and got noble prize in 1972. Large “Y” or “T” shaped protein produced by plasma cells to neutralize pathogens Antibody recognises a unique antigen via Fab’s (Fragment of Antigen Binding ) variable region Each tip of “Y” of an antibody contains a paratope which is specific to an epitope of Antigen

3. Structure of I mmunoglobulins Antibody ( immunoglobulin) - glycoproteins composed of one or more units, each containing four polypeptide chains T wo identical heavy chains (H) & two identical light chains (L). The amino terminal ends of the polypeptide chains show considerable variation in amino acid composition Each L chain consists of one variable domain, VL, and one constant domain, CL. (220 Aminoacids ) The H chains consist of a variable domain, VH, and three constant domains CH1, CH2 and  CH3. (440-550 Aminoacids ) Each heavy chain has about twice the number of amino acids and molecular weight (~50,000) as each light chain (~25,000), resulting in a total immunoglobulin monomer molecular weight of approximately 150,000Dts.

4. Heavy and light chains are held together by a combination of non-covalent interactions and covalent inter-chain disulfide bonds , forming a bilaterally symmetric structure. The V regions of H and L chains comprise the antigen-binding sites of the immunoglobulin (Ig) molecules. Each Ig monomer contains two antigen-binding sites and is said to be bivalent . The hinge region is the area of the H chains between the first and second Constant region domains and is held together by disulfide bonds . This flexible hinge (found in IgG, IgA and IgD, but not IgM or IgE) region allows the distance between the two antigen-binding sites to vary.

5. Hyper variable regions/Hot spots Present in both heavy and light chains variable regions The Light chain variable domain include three hypervariable regions present in between 23-34,50-56 and 89-97 amino acid moieties In heavy chains four hot spots present in between 1-23,35-49, 57-88 and 98-107 amino acid moieties These are brought together to form Paratopes Paratopes are very specific to Epitopes because of these For Eg . IgG produced in response to different antigens show variation in these hot spots only .

6. Hinge region It is an extend peptide sequence between Ch1 and CH2 domains of heavy chain Rich in cysteine and proline AAs , extremely variable in amino acid sequence It provides flexibility to antigen binding part Because of proline content, hinge is highly vulnerable to lytic enzymes viz. Papain, pepsin Cysteine helps in disulphide bond with opposite heavy chain Hinge is absent in IgM and IgE Instead they have an additional constant domain which acts like hinge

7. Disulphide bond Single covalent bond Between thiol groups of cysteine residues Also known as “ S-S bond ” or “ Disulphide bridge ” Play an important role in the folding and stability of some proteins secreted to EC medium Total number of disulphide bonds depend upon the number of domains in the polypeptide chains of Ig molecule.

8. Carbohydrate Moiety Significant amount of carbohydrate in CH2 region It is an oligosaccharide with variable monosaccharides May include mannose as in IgM or N-acetyl lactosamine as in IgG No glycosylation in Fab region Function is yet to know May help in passage through biological membranes

9. Classes of Immunoglobulins The five primary classes of immunoglobulins are IgG, IgM, IgA, IgD and IgE. These are distinguished by the type of heavy chain found in the molecule. IgGs have gamma-chains IgMs have mu-chains IgAs have alpha-chains IgEs have epsilon-chains and IgDs have delta-chains.

10. Differences in heavy chain polypeptides allow these immunoglobulins to function in different types of immune responses and at particular stages of the immune response. The polypeptide protein sequences responsible for these differences are found primarily in the Fc fragment. While there are five different types of heavy chains, there are only two main types of light chains: kappa (κ) and lambda (λ).

11. Antibody classes differ in valency as a result of different numbers of Y-like units (monomers) that join to form the complete protein. For example, in humans, functioning IgM antibodies have five Y-shaped units (pentamer) containing a total of 10 light chains, 10 heavy chains and 10 antigen-binding sites. IgA is a Dimer with four antigen binding sites IgG , IgD and IgE are monomers with two antigen binding sites

12. IgG IgG, a monomer, is the predominant Ig class present in human serum. Produced as part of the secondary immune response to an antigen, this class of immunoglobulin constitutes approximately 75% of total serum Ig. IgG is the only class of Ig that can cross the placenta in humans, and it is largely responsible for protection of the newborn during the first months of life. Because of its relative abundance and excellent specificity toward antigens, IgG is the principle antibody used in immunological research and clinical diagnostics.

13. The role of IgG in the immune response IgG is the major immunoglobulin in blood, lymph fluid, cerebrospinal fluid and peritoneal fluid and A key player in the humoral immune response. Serum IgG in healthy humans presents approximately 15% of total blood proteins The Fc portion of IgG, but not F(ab´)2 or Fab fragments, can cross the placenta of a mother and enter fetal circulation, providing the fetus with postpartum protection. IgG molecules are able to react with Fcγ receptors that are present on the surface of macrophages, neutrophils and natural killer cells, and can activate the complement system.

14. The binding of the Fc portion of IgG to the receptor present on a phagocyte is a critical step in the opsonization . Phagocytosis of particles coated with IgG antibodies is a vital mechanism that cells use to cope with microorganisms. 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.

15. IgG subclasses There are four IgG subclasses The subclasses differ in the number of disulfide bonds and the length and flexibility of the hinge region. Except for their variable regions, all immunoglobulins within one class share about 90% homology, but only 60% among classes. Determination of IgG subclasses can be a valuable tool in indicating a potential antibody deficiency. Selective IgG subclass deficiencies are associated with disease. In cases with prolonged or severe infections, determination of IgG levels can provide additional insight into the manifestation of disease. It is important to interpret IgG subclass concentrations in correlation to the donor's age since the immune system matures during childhood. Because of its relative abundance and excellent specificity toward antigens, IgG is the principle antibody used in immunological research and clinical diagnostics.

16. IgG class Properties of IgG : Molecular weight: 150,000 H-chain type (MW): gamma (53,000) Serum concentration: 10 to 16 mg/ mL Percent of total immunoglobulin: 75% Glycosylation (by weight): 3% Distribution: intra- and extravascular Function: secondary response

17. IgG1 IgG1 comprises 60 to 65% of the total IgG Predominantly responsible for the thymus-mediated immune response against proteins and polypeptide antigens. IgG1 binds to phagocytic cells and can activate the complement cascade. IgG1 immune response can already be measured in newborns and reaches its typical concentration in infancy. A deficiency in IgG1 isotype is typically a sign of a hypogammaglobulinemia .

18. IgG2 IgG2, the second largest of IgG isotypes, Comprises 20 to 25% of the main subclass The prevalent immune response against carbohydrate/polysaccharide antigens. “Adult” concentrations are usually reached by 6 or 7 years old. Among all IgG isotype deficiencies, a deficiency in IgG2 is the most common and is associated with recurring airway/respiratory infections in infants.

19. IgG3 IgG3 comprises around 5 to 10% of total IgG plays a major role in the immune responses against protein or polypeptide antigens. The affinity of IgG3 can be higher than that of IgG1.

20. IgG4 Comprising usually less than 4% of total IgG IgG4 does not bind to polysaccharides. In the past, testing for IgG4 has been associated with food allergies, and Elevated serum levels of IgG4 are found in patients suffering from sclerosing pancreatitis, cholangitis and interstitial pneumonia caused by infiltrating IgG4 positive plasma cells. The precise role of IgG4 is still mostly unknown.

21. IgM class Properties of IgM: Pentamer , Largest antibody Molecular weight: 900,000 H-chain type (MW): mu (65,000) Serum concentration: 0.5 to 2 mg/ mL Percent of total immunoglobulin: 10% Glycosylation (by weight): 12% Distribution: mostly intravascular Function: primary response Eliminates pathogens in early humoral immunity before sufficient IgG.

22. IgA class Properties of IgA: Dimer , Secretory antibody Molecular weight: (Alpha)320,000 (secretory) H-chain type (MW): alpha (55,000) Serum concentration: 1 to 4 mg/ mL Percent of total immunoglobulin: 15% Glycosylation (by weight): 10% Distribution: intravascular and secretions like saliva, tears, breast milk, mucosal area of gut, Respiratory tract, Urinogenital tract Function: protect mucus membranes and prevents pathogen colonization

23. IgD class Properties of IgD: Monomer Molecular weight: 185,000Dts Half life of 2-3 days H-chain type (MW): delta (70,000) Serum concentration: 0 to 0.4 mg/ mL Percent of total immunoglobulin: 0.25% Glycosylation (by weight): 13% Distribution: lymphocyte surface Function: 1.an antigen receptor on B-cells which are not exposed to antigens 2.it activates basophils and mast cells to produce antimicrobial factors

24. IgE class Properties of IgE: Discovered by Teruka and Kimishige Ishizaka in 1966 Scarce isotype found only in mammals Molecular weight: 190,000Dts Half life of 2-3 days H-chain type (MW): epsilon (73,000) Serum concentration: 10 to 400 ng / mL Percent of total immunoglobulin: 0.002% Glycosylation (by weight): 12% Distribution: basophils and mast cells in saliva and nasal secretions Function: protect against parasites and allergy.

25. Functions of Antibodies Neutralization Agglutination Precipitation Opsonization Complement activation(Fixation) Activation of Effector Cells Natural Antibodies

26. Neutralization Neutralizing antibodies block parts of the surface of bacterial cell or virion to render its attack ineffective

27. Agglutination “Glue together” foreign cells into clumps Clumps are attractive targets for phagocytosis

28. Precipitation Glue-together serum soluble antigens Forcing them to precipitate out of solution in clumps Clumps are attractive targets of phagocytosis

29. Complement activation Fixation in which antibodies latched to foreign cell Encourage complement to attack it with membrane attack complex which leads to Lysis of the foreign cell Encouragement of inflammation by chemotactically attracting inflammatory cells

30. Activation of Effector cells By coating the pathogen antibodies can activate effector functions like Phagocytosis Mast cells and neutrophils to degranulate Natural killer cells will release cytokines and cytotoxic molecules Ultimately results in destruction of invading microbe

31. Natural antibodies Humans and higher primates Defined as “antibodies produced without any previous infection, vaccination, other foreign exposure or passive immunization” Can activate classical complement pathway leads to lysis of enveloped virus particles prior to adaptive immunity Rejection of Xenotransplanted organs is the result of Natural antibodies

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