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ARTICLES
Year : 1972  |  Volume : 20  |  Issue : 2  |  Page : 39-41

Immunity


Haffkine Institute, Bombay-12, India

Correspondence Address:
S S Rao
Haffkine Institute, Bombay-12
India
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Source of Support: None, Conflict of Interest: None


PMID: 4668477

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How to cite this article:
Rao S S. Immunity. Indian J Ophthalmol 1972;20:39-41

How to cite this URL:
Rao S S. Immunity. Indian J Ophthalmol [serial online] 1972 [cited 2020 Apr 1];20:39-41. Available from: http://www.ijo.in/text.asp?1972/20/2/39/34668

The immune response is the most sophisticated defense mecha­nism which finds expression in all vertebrates including fishes, repti­les, birds and mammals. The immune mechanism is extremely versatile though it is characterised by great specificity as shown by its ability to recognize a single change of an amino acid in a protein or very subtle differences in structure of any substance such as protein, carbohydrate, lipid or even simple small molecules.

Three types of immunity are re­cognized. They are : (1) natural immunity, (2) humoral immunity, and (3) cellular immunity. The last two are concerned with the immune response and not the first. In natural immunity the resistance to an infectious disease is due to heritable genetic factors. An exam­ple is the resistance to malaria of persons who produce a detective hemoglobin in their red blood cells called sickle cell hemoglobin. Humoral immunity is due to the formation of unique proteins called `antibodies' which can be detected in blood, extravascular fluids and in secretions and are formed when any foreign substance usually of high molecular weight called 'anti­gen' is introduced into the body. The antibodies have the interest­ing property that they combine only with the antigen that was used to produce it and not any other substance. In cellular immnu­nity, the lymphocytes which form a part of the white blood cell population are themselves active and recognize a foreign antigen. This immunity is transferable only by the lymphocytes and not by the blood serum. In resistance to tuber­culosis or in the rejection of a foreign skin or organ graft such sensitised lymphocytes play the major role.

Organs and cells concerned with the immune response.

The development of the immune response must have started about 400 million years ago when the vertebrates evolved. This is a fairly recent event in the life of our earth. The special organs in the vertebrates concerned with the immune response are: thymus, bone marrow, spleen, lymphnodes and in birds there is a special lympha­tic organ called the bursa of Fabricius situated near the end of the alimentary canal. The cells concerned are the lymphocytes and the plasma cells. The role of the thymus (which is situated just above the heart) and the bursa in the immune response was not known till about ten years ago. It was then established that thymec­tomy of new born chick resulted in normal humoral (antibody) res­ponse but very low cellular res­ponse resulting in the acceptance of a homograft. Neonated bursec­tomy resulted in deficiency of plasma cells which secrete the antibody but the celluar response was adequate as shown by normal rejection of skin transplant in mammals it is believed that the bone marrow is the equivalent of the bursa of birds.

Thymus and the bone marrow release a large number of lymph­ocytes into the lymphatic and blood circulation. Most of these cells, however, have a short half life of about 3 days and by themselves are unable to recognize an antigen or produce an antibody. The thy­mus and marrow cells interact in the spleen and lymph nodes to pro­duce clones of lymphocytes which can recognize any foreign antigen It has not been possible so far to duplicate this in vitro. It has re­cently been established that the antigen recognizing lymphocytes carry on their surface receptor sites which resemble the antigen com­bining sites of antibodies. Such lymphocytes are stimulated by their specific antigen to divide and those of thymic origin give rise to cells responsible for cellular immu­nity and those of bone marrow origin give rise to plasma cells which produce antibodies respon­sible for humoral immunity.

The thymus, in addition to pro­ducing lymphocytes also secretes a hormone called 'thymosin' which stimulates development of spleen and lymphnodes. Neonatal thy­mectomy impairs the development of spleen and lymphnodes which can be made up by injecting the 'thymosin'.

The role of thymus in human immune response was confirmed when Dr George discovered in 1965 a syndrome named after him where the newborn suffered from tetany. There was aplasia of thy­mus, and lymphocyte count was less than the normal. The plasma cells were present and antibody production was also nearly normal. The infant could accept skin graft. This syndrome is due to a genetic defect of an autosomal recessive gene. In the agammaglobulinemia of Burton, the thymus is normal, the patient shows rejection of homograft, the number of circulat­ing lymphocytes are normal, but there are no plasma cells and the patient is very susceptible to infectious diseases because of the inability to produce antibody I globulin. This defect has been traced to a recessive gene in the sex chromosome.

All the events that lead to an immune response after the intro­duction of a foreign antigen have not yet been clarified. However, from the facts that have been so far gathered, the following sequ­ence can be formulated. In the foetal life and for some time in the neonatal existance, the interaction between the thymus lymphocytes and marrow or bursa lymphocytes continue in spleen and lymph nodes producing clones of lymph­ccytes which are sensitive to any substance which has a structure not present in the animal. It is presumed that the cells capable of reacting with self antigens are de­stroyed during this time so that the immunologically mature animal reacts only with the non-self anti­gen. In a mature animal when a foreign antigen is introduced the antigen is presumably engulfed by the macrophages which take them to the spleen or lymph node where it transfers the antigen to the particular lymphocyte which has a receptor site for the antigen the lymphocyte is then stimulated, to multiply and produce a large number of daughter cells. Some of these daughter cells mature into short lived plasma cells which secrete the antibody which can combine specifically with the anti­gen. Some of the daughter cells are themselves capable of combin­ing with the antigen and are res­ponsible for the cellular response.

The subject immunology was confined in the early years to the development of resistance to in­fectious diseases. Vaccination using killed or attenuated microor­ganisms was practised to build up antibodies which could overcome the infectious agent. However, the scope of immunology widened very much when it was discovered about 20 years ago that allergy, hypersen­sitivity, transfusion reactions, re­jection of homo or heterograft, immediate and delayed hypersen­sitivity to small or large molecules, diseases now grouped under auto­immune diseases, resistance or susceptibility to spontaneous or transplanted tumours are all mani­festations of the same immune mechanism. Immunology is now one of the fastest developing sci­ences. The advances made in this subject in recent years and the development of special techniques have found application in almost every branch of medical research. There is now need for the study of immunology as a separate subject in the curriculum of the medical colleges. It is impossible to review the recent advances in all aspects of Immunology. Advances in only two fields will be discussed in brief in the next two articles.




 

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