ANTIBODY - DEFINITION, STRUCTURE, FUNCTIONS & CLASSIFICA....
STRUCTURE OF ANTIBODY

ANTIBODIES – DEFINITION, STRUCTURE, FUNCTIONS & CLASSIFICATION

DEFINITION OF ANTIBODY

An antibody is defined as a humoral protein that is immunoglobulin produced in response to an antigenic stimulus, with which antigen reacts specifically and in an observable manner.



GENERAL PROPERTIES OF ANTIBODIES

⇒ Antibodies serve as a protective agent against organism or foreign bodies.

⇒ Antibodies are found in serum, length and other body fluids.

⇒ Secreted antibodies circulate in the blood, where they eliminate or neutralize the antigen by their effector functions such as phagocytosis, antibody dependent cell mediated cytotoxicity that is ADCC, opsonization etc.

⇒ Antibodies are proteins in nature. These are chemically indistinguishable from normal Gamma globulin. The term immunoglobulin is used to describe these closely related proteins.

⇒ A serum having high antibody levels following infection or immunization is called immune Sera.

⇒ Immunoglobulins are synthesized by plasma cells and to some extent by lymphocytes

⇒ These make 20 to 25% of the total serum proteins. All antibodies are Immunoglobulins, but all Immunoglobulins may not be antibodies.

FUNCTIONS OF ANTIBODIES / IMMUNOGLOBULINS

The main biological functions of Immunoglobulins are as follows – 

Complement activation The complement system is an unspecified defence mechanism of the immune system. It has a role in a large number of inflammatory and cytotoxic reactions and macrophage activation

Agglutination Agglutination of bacteria and viruses is another biological activity of the Immunoglobulins, above all of the immunoglobulin M i.e. IgM.

Antibody dependent cellular cytotoxicity i.e. ADCC ADCC-Cytotoxic phenomena are not only induced by TCD 8 + Lymphocytes, other immune cells, such as macrophages or Natural Killer (NK) cells, can also destroys cells with the cooperation of antibodies.

Neutralization Some immunoglobulin isotypes such as IgG, IgM and IgA are able to bind toxin, bacteria or virus is and neutralize their activity.  In case of viruses, the neutralization process allows antibodies to prevent viruses from infected cells, due to the fact that antibodies coat the virus fragments needed for it to bind to the Cell.

Opsonization – This is the phenomenon by which the antibodies enwrap an antigen (Virus, Bacteria etc.) and activate phagocytosis by means of FC fragments of macrophages and Neutrophils or polymorphnuclear leukocytes.

Mucosal protection – IgA protect the epithelial surfaces from infectious agent. Its dimer or tetramer conformation allows having 4 to 8 antigen binding sites. This makes it a tremendously effective against different bacterial antigens; by means of ADCC reactions. IgA is not bactericidal but it has a great capability to neutralize some viruses.

STRUCTURE OF IMMUNOGLOBULINS

⇒ An antibody molecule consists of two identical heavy and two identical light chains. The “Heavy” or ‘H’ chains are longer and “Light” or ‘L’ chains are shorter. Both the types of chains are polypeptide in nature.

⇒ The two heavy chains are held together by disulphide Bond. Each light chain is also attached to heavy chain by disulfide Bond.

⇒ The ‘L’ chains are similar in all classes of Immunoglobulins. They are present in two forms Kappa (k) and Lambda (λ). Each immunoglobulin has either two kappa or two lambda light chains but both (k & λ) are never found together in a molecule.

⇒ Based on their size, carbohydrates content and amino acid analysis, five groups if Ig distinguished. The H chains are structurally and antigenically different for each immunoglobulin it is indicated by Greek letter as:-

⇒ The shape of Ig was first described by Rodney Robert Porter, Gerald Edelman, Alfred Nisonoff and their colleagues.

⇒ Rabbit IgG antibody to egg albumin, digested with the aid of papain in the presence of cysteine, splits into two fractions – An insoluble fraction which crystallizes in cold (referred to as Fc for crystallizable) and a soluble fragment which whilst unable to precipitate with egg albumin, can nevertheless bind to it. This fragment is named as the Fab (antigen binding).

⇒ The Fab area of Ig binds with t epitope of antigen & Fc place binds with other proteins.

⇒ Fab fragment is split into areas – variable area & constant area. The variable area modifies in line with an epitope of antigen; if antibody recognizes the epitope.

TYPES OF IMMUNOGLOBULINS / ANTIBODIES

Immunoglobulin G i.e. IgG

  • It is the major serum immunoglobulin about 80% of total amount. The normal serum concentration is about 8 to 16 mg/ml.
  • It is the only immunoglobulin that is transported through Placenta and provides natural passive immunity to the infant.
  • It is distributed equally between the intravascular and extravascular compartments.
  • Immunoglobulin G appears late but persists for a longer period. It appears after the initial immune response which is immunoglobulin M in nature.
  • It participates in precipitation, complement fixation and neutralization of toxin and viruses
  • Immunoglobulin G binds to microbes and enhances the process of phagocytosis.
  • It is protected against those microbes which are active in the blood and tissues.




Immunoglobulin A i.e. IgA

  • Immunoglobulin A is second major serum immunoglobulin constitutes about 10 to 13% of serum immunoglobulin. The normal serum concentration is 0.6 to 4.2 mg/ml.
  • Immunoglobulin A occurs in two forms – serum IgA and secretory IgA.
  • Serum immunoglobulin A is a monomeric molecule, while Immunoglobulin A found on mucosal surfaces and in secretion is a dimer.
  • Immunoglobulin A is the principal immunoglobulin present in secretions such as Milk, Saliva, Tears, Sweat, Nasal fluids, Colostrum, and in secretions of Respiratory, Intestine and Genital systems.
  • Immunoglobulin A is mainly synthesized locally by plasma cells and little is derived from serum.

Immunoglobulin M i.e. IgM

  • It constitutes about 5 to 8% of total serum immunoglobulin. The normal level in serum is 0.5 to 2 mg/ml.
  • It is a heavy molecule with a molecular weight 900K to 1000K hence also called the millionaire molecule.
  • Immunoglobulin M is mainly distributed intravascularly which is about 80%.
  • It is the earliest synthesized immunoglobulin by fetus – in about 20 weeks of age.
  • It appears early in response to infection before immunoglobulin G. Immunoglobulin M antibodies are short lived and disappear earlier than immunoglobulin G. Hence, its presence in serum indicates recent infection
  • It cannot cross the Placenta; presence of immunoglobulin and antibodies in serum of newborn indicates congenital infection. Its detection is therefore useful for the diagnosis of congenital syphilis, HIV infection, and rubella etc.
  • Immunoglobulin M provides protection against blood invasion of microorganisms. Its deficiency is often associated with septicemia.

Immunoglobulin D i.e. IgD

  • Immunoglobulin D is present in a concentration of 3 mg/dl in serum. It is mostly intravascular in distribution.
  • Like immunoglobulin M, it is also present on the surface of unstimulated B Lymphocytes in blood and act as recognition receptor for antigens.
  • Combination of cell membrane bound immunoglobulin D with the corresponding antigen leads to specific stimulation of these B Lymphocytes – either by activation and cloning to form antibody or by suppression.

Immunoglobulin E i.e. IgE

  • Immunoglobulin E is mainly produced in the lining of respiratory and intestinal tract.
  • Serum contains only traces of a few nanograms/ml. It is mostly distributed extravascularly.
  • It is also referred to as reagins.
  • It is heat labile that is inactivated at 56° Celsius in one hour whereas other Immunoglobulins are heat stable.
  • It has affinity for surface of tissue cells, particularly mast cells of the same species.
  • High level in serum is seen in children with a high load of intestinal parasitism.
  • Immunoglobulin E mediates Type 1 hypersensitivity reaction this is responsible for Asthma, Hay fever and Eczema etc.

THE ROLE OF VARIOUS TYPES OF IMMUNOGLOBULINS IN OUR BODY

  • Immunoglobulin G protects the body Fluids.
  • Immunoglobulin A protects the body surfaces.
  • Immunoglobulin M protects the bloodstream.
  • Immunoglobulin D is the recognition molecule on surface of B Lymphocytes.
  • Immunoglobulin E mediates the reaginic hypersensitivity.



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