Antibodies are glycoproteins which are highly specific to antigens. They are also known as immunoglobulins. Antibodies are produced by plasma cells which in turn are formed by B-lymphocytes. The plasma cell produces about 2000 molecules of antibodies per sec.
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| Structure of Antibody |
Antibody is a 'Y' shaped structure. It consists of four polypeptide chains, two heavy or H chains and two light or L chains. The four polypeptide chains are held together by disulfide bonds to form a 'Y' shaped structure. The region holding arms and stem of antibody is termed as hinge.
Each chain of the antibody includes two distinct regions, the variable region and the constant region. Variable regions Constitute the antigen-binding site. This part of antibody recognizes and binds to the specific antigen forming an antigen-antibody complex. Since most antibodies carry two antigen binding sites, they are said to be bivalent.
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Antigen -antibody complex:
A branch of immunology which deals with the study of antigen-antibody interaction is called serology. Each antibody is specific for a particular antigen.
Combining sites of antigen, called antigenic determinants react with the corresponding antigen binding sites of antibodies called paratopes. The antigen binding sites are located on the variable regions of the antibody.
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 systems like ABO, Rh, Duffy, Kidd, Lewis, P, MNS etc.
* In ABO system the blood groups are determine by the presence or absence of antigen A and antigen B, the blood group of person is classified into four groups A, B, AB and O.
*Blood group A:
Individuals with blood group 'A' have the A antigen on the surface of their red blood cells and 'b' antibodies in their plasma.
*Blood group B:
Individuals with blood group 'B' have the B antigen on the surface of their RBCs and 'a' antibodies 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'Brien lack A and B antigens on the surface of their RBCs and show presence of both 'a' and 'b' antibodies in their plasma.
During blood transfusion only the compatible blood can be used. The one who gives blood is called the donor and the recipient is the one who receives blood. Individuals with blood group O can donate blood to anyone, while those individuals with blood group AB can receive blood from any person.
Rh factor:
It is an antigen present on surface of red blood cells. It was first discovered by Landsteiner and Wiener(1940) on the surface of RBCs of Rhesus monkey, so it was named as Rhesus or Rh factor. .
Person having Rh factor (D antigen) are called Rh positive and those lacking D antigen are called Rh negative.
Rh factor is important in blood transfusion. Rh antigen induces a strong immunogenic response when introduced into Rh-ve individuals. It results in haemolytic disease of the newborn, which is also called erythroblastosis foetalis.