10 Human Health and Diseases - part 02 - Structure of Antibody

 


10 Human Health and Diseases - part 02 - Structure of Antibody

Structure of Antibody:

  • Antibodies are glycoproteins which are highly specific to specific antigens. 
  • Known as Immunoglobulins (Igs), produced in response to antigenic stimulation.
  • Produced by plasma cells which in turn are formed by B–lymphocytes. 
  • Mature plasma cells produce antibodies at an extremely rapid rate i.e. about 2000 molecules per second.

Structure:
  • Antibody is a ‘Y’ shaped molecule. 
  • Each immunoglobulin molecule is made up of four polypeptide chains. 
  • There are two heavy or H-chains and two light or L-chains. 
  • Four polypeptide chains are held together by disulfide bonds (-s-s-) to form a ‘Y’ shaped structure. 
  • Region holding together arms and stem of antibody, is termed as hinge. 
  • Each chain of the antibody includes two distinct regions -  
  1. Variable region 
  2. Constant region. 
  • Variable regions constitute the antigen binding site (paratope). 
  • This part of antibody recognizes and binds to the specific antigen to form an antigen-antibody complex. 
  • Since most antibodies carry two antigen binding sites, they are said to be bivalent.
Formation of antigen-antibody complex:

  • Study of antigen-antibody interactions is called serology. 
  • Each antibody is specific for a particular antigen. 
  • Combining sites of antigen, called antigenic determinants (epitopes) react with the corresponding antigen binding sites of antibodies called paratopes. 
  • The antigen binding sites (paratopes) are located on the variable regions of the antibody. 
  • Small variations in the variable regions make each antibody highly specific for a particular antigen. 
  • The variable region enables the antibody to recognize the specific antigen and bind to specific antigen in a lock and key manner forming an antigen-antibody complex.

Antigen on Blood Cells:
  • There are several known antigens on the surface of human red blood cells. 
  • These antigens give rise to different blood groups.
  • There are many genetically determined blood groups system like ABO, Rh, Duffy, Kidd, Lewis, P, MNS, Bombay blood group, etc.

ABO Blood Groups:

  • A, B and O blood groups were discovered by Karl Landsteiner in 1900. 
  • Blood group AB was discovered by Landsteiner’s students Decastello and Sturli in 1902. 
  • Landsteiner was awarded the Nobel prize for his discovery of human blood groups.
  • He found two antigens or agglutinogens on the surface of human red blood cells and named them as antigen A and antigen B. 
  • He also noticed the corresponding antibodies or agglutinins in the serum called ‘a’ and ‘b’.
  • In ABO system, the blood groups are determined by the presence or absence of antigen A and antigen B.
  • Blood group of person is classified into four groups A, B, AB and O.
Blood group A : 
  • Individuals, with blood group ‘A’ have the antigen A on the surface of their red blood cells (RBCs) and antibody ‘b’ in their plasma.
Blood group B : 
  • Individuals with blood group ‘B’ have the antigen B on the surface of their RBCs and antibodies ‘a’ in their plasma.
Blood group AB : 
  • Individuals with blood group ‘AB’ have both antigens A and B on the surface of their RBCs and no antibodies in their plasma.
Blood group O – 
  • Individuals with blood group ‘O’ lack both antigens A and B on the surface of their RBCs and show presence of both ‘a’ and ‘b’ antibodies in their plasma.
Rh factor:
  • Rh is the most complex of the blood group system. 
  • Rh –factor is an antigenic protein present on the surface of the red blood cells in the human beings. 
  • Discovered by Landsteiner and Wiener (1940), on the surface of RBCs of Rhesus monkey, so it is called Rhfactor (also called D antigen). 
  • Person having Rh factor (D antigen) are called Rh positive (Rh +ve) and those lacking D antigen are called Rh negative (Rh -ve).
  • Rh (D) antigen induces a strong immunogenic response when introduced into Rh-ve individuals. 
  • Rh blood group is an important factor in blood transfusion and is involved in haemolytic diseases of the newborn (HDN), which is called erythroblastosis foetalis (destruction of the erythrocytes of the foetus). 
  • It occurs when an Rh -ve mother conceives Rh+ve foetus.
  • The Rh +ve RBCs from the foetus may enter the mother’s circulatory system during child birth, causing her to produce anti-Rh antibodies. 
  • As a result, subsequent Rh+ve foetuses will be exposed to the anti-Rh antibodies produced by mother, which result in HDN. 
  • In order to prevent HDN, Rh -ve mother is injected with the anti-Rh antibody during all pregnancies carrying Rh +ve foetus.

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