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Immunity | Human Immune System

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Immunity – Innate Immunity – Acquired Immunity, Active – Passive Immunity. Vaccination and Immunization, Allergies, Auto Immunity, Immune System in the Body.

  • Immunity is of two types: (i) Innate immunity and (ii) Acquired immunity.

Innate Immunity

  • Innate immunity is non-specific type of defense, that is present at the time of birth.
  • Innate immunity is accomplished by providing different types of barriers to the entry of the foreign agents into our body.

Innate immunity consist of four types of barriers. These are —

  • Physical Barriers: Skin on our body is the main barrier which prevents entry of the micro-organisms. Mucus coating of the epithelium lining the respiratory, gastrointestinal and urogenital tracts also help in trapping microbes entering our body.
  • Physiological Barriers: Acid in the stomach, saliva in the mouth, tears from eyes-all prevent microbial growth.
  • Cellular Barriers: Certain types of leukocytes (WBC) of our body like polymorpho-nuclear leukocytes (PMNL-neutrophils) and monocytes and natural killer (type of lymphocytes) in the blood as well as macrophages in tissues can phagocytose and destroy microbes.
  • Cytokine Barriers: Virus-infected cells secrete proteins called interferons which protect non-infected cells from further viral infection.

Acquired Immunity

  • Acquired immunity is pathogen specific. It is characterized by memory. This means that our body when it encounters a pathogen for the first time produces a response called primary response which is of low intensity.
  • Subsequent encounter with the same pathogen elicits a highly intensified secondary or anamnestic response. This is ascribed to the fact that our body appears to have memory of the first encounter.
  • The primary and secondary immune responses are carried out with the help of two special types of lymphocytes present in our blood, i.e., B-lymphocytes and T-lymphocytes.
  • The B-lymphocytes produce an army of proteins in response to pathogens into our blood to fight with them. These proteins are called Antibodies [a blood protein produced by the body in response to and counteracting an antigen].
  • The T-cells themselves do not secrete antibodies but help B cells produce them.
  • Each antibody molecule has four peptide chains, two small called light chains and two longer called heavy chains. Hence, an antibody is represented as H2L2.
  • Different types of antibodies are produced in our body. IgA, IgM, IgE, IgG are some of them.
  • Because these antibodies are found in the blood, the response is also called as humoral immune response. This is one of the two types of our acquired immune response – antibody mediated. The second type is called cell-mediated immune response or cell mediated immunity (CMI). The T-lymphocytes mediate CMI.
  • Very often, when some human organs like heart, eye, liver, kidney fail to function satisfactorily, transplantation is the only remedy to enable the patient to live a normal life. Then a search begins – to find a suitable donor. Why is it that the organs cannot be taken from just anybody? What is it that the doctors check?
  • Grafts from just any source – an animal, another primate, or any human beings cannot be made since the grafts would be rejected sooner or later. Tissue matching, blood group matching are essential before undertaking any graft/transplant and even after this the patient has to take immuno-suppresants all his/her life. The body is able to differentiate ‘self’ and ‘nonself’ and the cell-mediated immune response is responsible for the graft rejection.

Active and Passive Immunity

  • When a host is exposed to antigens [a substance which the body recognizes as alien and which induces an immune response], which may be in the form of living or dead microbes or other proteins, antibodies are produced in the host body. This type of immunity is called active immunity.
  • Active immunity is slow and takes time to give its full effective response. Injecting the microbes deliberately during immunization or infectious organisms gaining access into body during natural infection induce active immunity.
  • When ready-made antibodies are directly given to protect the body against foreign agents, it is called passive immunity.
  • Do you know why mother’s milk is considered very essential for the newborn infant? The yellowish fluid colostrum secreted by mother during the initial days of lactation has abundant antibodies (IgA) to protect the infant.
  • The foetus also receives some antibodies from their mother, through the placenta during pregnancy. These are some examples of passive immunity.

Vaccination and Immunization

  • The principle of immunization or vaccination is based on the property of ‘memory’ of the immune system.
  • In vaccination, a preparation of antigenic proteins of pathogen or inactivated/weakened pathogen (vaccine) are introduced into the body.
  • The antibodies produced in the body against these antigens would neutralize the pathogenic agents during actual infection.
  • The vaccines also generate memory – B and T-cells that recognize the pathogen quickly on subsequent exposure and overwhelm the invaders with a massive production of antibodies.
  • If a person is infected with some deadly microbes to which quick immune response is required as in tetanus, we need to directly inject the preformed antibodies, or antitoxin (a preparation containing antibodies to the toxin).
  • Even in cases of snakebites, the injection which is given to the patients, contain preformed antibodies against the snake venom. This type of immunization is called passive immunization.
  • Recombinant DNA technology has allowed the production of antigenic polypeptides of pathogen in bacteria or yeast. Vaccines produced using this approach allow large scale production and hence greater availability for immunization, e.g., hepatitis B vaccine produced from yeast.


  • Did this happen to you? When you have gone to a new place and suddenly you started sneezing, wheezing for no explained reason, and when you came away, your symptoms disappeared?
  • Some of us are sensitive to some particles in the environment. The above-mentioned reaction could be because of allergy to pollen, mites, etc., which are different in different places.
  • The exaggerated response of the immune system to certain antigens present in the environment is called allergy. The substances to which such an immune response is produced are called allergens. The antibodies produced to these are of IgE type.
  • Common examples of allergens are mites in dust, pollens, animal dander, etc.
  • Symptoms of allergic reactions include sneezing, watery eyes, running nose and difficulty in breathing.
  • Allergy is due to the release of chemicals like histamine and serotonin from the mast cells.
  • For determining the cause of allergy, the patient is exposed to or injected with very small doses of possible allergens, and the reactions studied.
  • The use of drugs like anti-histamine, adrenalin and steroids quickly reduce the symptoms of allergy.
  • Somehow, modern-day life style has resulted in lowering of immunity and more sensitivity to allergens – more and more children in metro cities of India suffer from allergies and asthma due to sensitivity to the environment. This could be because of the protected environment provided early in life.

Auto Immunity

  • Memory-based acquired immunity evolved in higher vertebrates based on the ability to differentiate foreign organisms (e.g., pathogens) from self-cells.
  • While we still do not understand the basis of this, two corollaries of this ability have to be understood.
  • One, higher vertebrates can distinguish foreign molecules as well as foreign organisms. Most of the experimental immunology deals with this aspect.
  • Two, sometimes, due to genetic and other unknown reasons, the body attacks self-cells. This results in damage to the body and is called auto-immune disease.
  • Rheumatoid arthritis which affects many people in our society is an auto-immune disease.

Immune System in the Body

  • The human immune system consists of lymphoid organs, tissues, cells and soluble molecules like antibodies. As you have read, immune system is unique in the sense that it recognizes foreign antigens, responds to these and remembers them. The immune system also plays an important role in allergic reactions, auto-immune diseases and organ transplantation.
  • Lymphoid Organs: These are the organs where origin and/or maturation and proliferation of Lymphocytes
  • The primary lymphoid organs are bone marrow and thymus where immature lymphocytes differentiate into antigen-sensitive lymphocytes.
  • After maturation the lymphocytes migrate to secondary lymphoid organs like spleen, lymph nodes, tonsils, Peyer’s patches of small intestine and appendix.
  • The secondary lymphoid organs provide the sites for interaction of lymphocytes with the antigen, which then proliferate to become effector cells.
  • The bone marrow is the main lymphoid organ where all blood cells including lymphocytes are produced.
  • The thymus is a lobed organ located near the heart and beneath the breastbone. The thymus is quite large at the time of birth but keeps reducing in size with age and by the time puberty is attained it reduces to a very small size.
  • Both bone-marrow and thymus provide micro-environments for the development and maturation of T-lymphocytes.
  • The spleen is a large bean shaped organ. It mainly contains lymphocytes and phagocytes. It acts as a filter of the blood by trapping blood-borne micro-organisms. Spleen also has a large reservoir of erythrocytes.
  • The lymph nodes are small solid structures located at different points along the lymphatic system. Lymph nodes serve to trap the micro-organisms or other antigens, which happen to get into the lymph and tissue fluid.
  • Antigens trapped in the lymph nodes are responsible for the activation of lymphocytes present there and cause the immune response.
  • There is lymphoid tissue also located within the lining of the major tracts (respiratory, digestive and urogenital tracts) called mucosal-associated lymphoid tissue (MALT). It constitutes about 50 per cent of the lymphoid tissue in human body.

Lymph Nodes - Immunity - Human Immune System.png

Picture Credits: Wikipedia


  • Health is not just the absence of disease. It is a state of complete physical, mental, social and psychological well-being.
  • Diseases like typhoid, cholera, pneumonia, fungal infections of skin, malaria and many others are a major cause of distress to human beings.
  • Vector-borne diseases like malaria especially one caused by Plasmodium falciparum, if not treated, may prove fatal.
  • Our immune system plays the major role in preventing these diseases when we are exposed to disease-causing agents.
  • The innate defenses of our body like skin, mucous membranes, antimicrobial substances present in our tears, saliva and the phagocytic cells help to block the entry of pathogens into our body.
  • If the pathogens succeed in gaining entry to our body, specific antibodies (humoral immune response) and cells (cell mediated immune response) serve to kill these pathogens.
  • Immune system has memory. On subsequent exposure to same pathogen, the immune response is rapid and more intense. This forms the basis of protection afforded by vaccination and immunization.

Source: NCERT Science Textbooks Class 6-12

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