PROPERTIES OF IMMUNOGEN AND IMMUNOGLOBULIN

BY

Smt Shilpa Bandrad

IMMUNOGEN

• Antigen is a substance which when introduced parentally into the body stimulates the production of an antibody with which it reacts specifically and in an observable manner.

ANTIGENIC DETERMINANT

​

​

​

• As most antigens are protein

in nature, they are present in

the folded, three dimensional, tertiary structure. Hence, there may be a cluster of aminoacid sequences on the three dimensional structure constituting a series of epitopes.

• Each of the epitope cluster is called an Antigenic determinant.

CLASSIFICATION OF ANTIGENS

1. Based on Immunogenicity-
1. Complete antigen- substances which can induce antibody formation by themselves and react specifically with these antibodies.
2. Incomplete antigen (Haptens)- substance unable to induce antibody formation on its own but can become immunogenic when covalently linked to proteins, called carrier protein. They are of two types-
1. Complex
2. simple

II. Based on origin-

1. Exogenous antigens- are antigens that have entered the body from the outside.
2. Endogenous antigens- are antigens that hae been generated within previously normal cells as a result of normal cell metabolism, or because of viral or intracellular bacterial infection.
3. Autoantigens- are the ‘self’ proteins or nucleic acids that due to some genetic or alterations get attacked by their own immune system causing autoimmune diseases.

PROPERTIES OF ANTIGENS

• Should be a foreign substance to induce an immune response.
• Molecular mass of 14,000 to 6,00,000 Da.
• Are mainly proteins and polyacharide.
• The more chemically complex they are, the more immunogenic they will be.
• Antigens are species- specific.
• The age influences the Immunogenicity. Very young and very old people exhibit very low immunogenecity.

IMMUNOGLOBULIN

• Antibodies are substances that are formed in the serum and tissue fluids in response to the presence in the body of an antigenic and react with it specifically in some observable manner.
• The antibody is produced by the immune system.
• Sera having high antibody levels are known as “immune sera”.

PROPERTIES OF ANTIBODIES

• Chemically antibodies are globulins and are therefore named as “Immunoglobulins”.
• The globulins are seperated into water soluble Pseudoglobulins and water insoluble Euglobulins.
• Most of the antibodies molecules are sedimented at 7s.
• Molecular weight around 1,50,000.
• Immunoglobulins are synthesized mainly by plasma cells.

• Some heavier antibodies known as macroglobulins or M-globulins, and are sedimented at 19s and have molecular weight around 9,00,000.
• Besides antibodies, globulins, immunoglobulins include abnormal proteins found in myeloma, cryoglobulin and the macroglobulin.
• All antibodies are immunoglobulins, but all immunoglobulin are not antibodies.
• 20-25% of the total serum protein are immunoglobulins.

STRUCTURE OF ANTIBODIES

• All antibodies have the

same basic molecular

structure.

• Immunoglobulin appears

as Y- shaped structure.

• Made up of two pairs of

polypeptide chains.

• One pair is small and is

called light chain, while

the other pair is long and

termed as heavy chain.

• The chains are held

together by disulphide bonds.

• LIGHT CHAIN (L-chain)-
• L-chain has mol.wt of ≈ 25,000
• Each L chain is made up of about 210-230 aminoacids.
• L chain is attached to the H chain by a disulphide bond.
• There are two types of L chains : Kappa (K) & Lambda (λ).
• L chains are structurally and chemically similar in all classes of Immunoglobulins.

• HEAVY CHAIN (H-Chain)-
• H chain has mol.wt of ≈ 50,000.
• Each H chain is made up of 420-460 aminoacids.
• Two H chains are joined together by one to five S-S bonds.
• H chains are structurally and antigenically different for each class of Immunoglobulin.

CLASSIFICATION OF IMMUNOGLOBULIN

1. IMMUNOGLOBULIN –G (IgG)-
1. It is major serum immunoglobulin

and accounts for about 70-80%

of the total immunoglobulin in

human serum.

• Normal serum conc is 8-16mg/ml
• Molecular wt – 1,50,000.
• Sedimentation co-efficient – 7S.
• Four subclasses- IgG1(65%), IgG2

(23%), IgG3(8%), IgG4 (4%).

​

• Half life – 23 days.
• Equally distributed between the intravascular and extravascular compartment.
• Contains less carbohydrate than other immunoglobulins.
• Catabolic rate varies with its serum concn. When its level is raised as in chronic malaria, Kala azar or Myeloma, IgG is rapidly catabolised in hypogammaglobulinemia.
• The IgG given for treatment will be catabolised slowly.

2. IMMUNOGLOBULIN A (IgA)-

• It is the second most abundant immunoglobulin present in serum – 10-13%.
• It is the predominant immunoglobulin in external

secretions such as breast milk, saliva, tears & mucous of bronchial, genitourinary & digestive tracts.

• IgA is of two types-
• Serum IgA-
• It is monomer containing 2H & 2L chains.
• Molecular wt – 1,60,000
• Sedimentation co-efficient – 7S
• Two subclasses – IgA1 & IgA2

​

b) Secretary IgA-

• It is found on the mucosal

surfaces & secretions of

respiratory, gastrointestinal &

genitourinary tracts.

• It is a dimer formed by two

monomer units joined together by a

glycoprotein termed as ‘J’ chain.

• Molecular wt – 3,85,000.
• Sedimentation co-efficient – 7S.
• It contains a glycine – rich polypeptide called S piece or secretary piece or secretary component.
• IgA inhibits the adherence of m.o’s to the surface of mucosal cells by covering the organs & thereby preventing their entry into the body tissue.

3. IMMUNOGLOBULIN M (IgM)-

• It is the first immunoglobulin

class produced in a primary

immune response.

• It is also the first immunoglobulin

to be synthesized by the fetus at

about 20 weeks of age.

• It accounts for about 5-10% of the

total serum immunoglobulin.

• Normal serum concn – 0.5 – 2% mg/ml.
• Its mol.wt is 9,70,000 & hence it is known as ‘millionaire molecule’.
• Sedimentation co-efficient – 19S.
• Half life – 5 days
• It is a pentamer consisting of 5 immunoglobulin subunits & a chain that joins the subunits ( J units).

• Nearly 80% of IgM is intravascular in distribution.
• Because of its high valency, it is highly efficient in agglutination, complement fixation & cytolytic activity.
• Since it is not transported across placenta, the presence of IgM in the fetus or newborn indicates intrauterine infection & is thus useful in the diagnosis of congenital infections such as syphilis, rubella, HIV infection & taxolasmosis.
• It is more efficient than IgG in human hemolysis, opsonisation & batericidal action but less efficient in the neutralization of viruses & toxins.

4. IMMUNOGLOBULIN D (IgD)-

• It is present in a concn of

3mg/100ml of serum & is

mostly intravascular.

• It is monomer & resembles IgM.
• Mol. Wt – 1,84,000.
• Sedimentation co-efficient – 7S.
• Half life – 3 days.
• Occur on the surface of unstimulated B-lymphocytes & serve as recognition receptors for antigen.
• Combination of cell membrane bound IgD with the corresponding antigen leads to specific stimulation of B-cell, either activation & cloning to form antibody or suppression.

​

​

5. IMMUNOGLOBULIN E (IgE)-

• Occurs in a very low concn in serum.
• It is monomer & resembles IgG

structurally.

• Mol. Wt – 1,88,000.
• Sedimentation co-efficient – 8S.
• Half life – 2 days.
• Mostly extravascular in distribution.
• It is heat-labile & inactivated at 56⁰C.
• Susceptible to mercaptoethanol.
• Elevated levels of serum are seen in conditions like asthama, hay fever & eczema.
• High IgE level in serum is seen in children with high load of intestinal parasites.
• It is responsible for anaphylactic type of hypersensitivity.

​

​

THANK YOU