• By massive immunization there is complete eradication of disease like smallpox.
  • Diseases like polio, diphtheria, pneumonia, and tetanus have been controlled in large extent.


  • The overall ability of the host to fight the disease causing organism by immune system is called immunity.
  • There are two types of immunity:
    • Innate Immunity.
    • Acquired Immunity.


Innate (non-specific) immunity:

  • Called inborn immunity.
  • Always available to protect out body.
  • This is called the first line of defense.
  • Consists of various barriers that prevent entry of foreign agents into the body.
  • If enters they are quickly killed by some other components of this system.
  • Different types of barriers are as follows:

Physical barriers:

  • Skin is the main barrier which prevents entry of micro-organism.
  • Mucous coating of the epithelium lining of respiratory, gastrointestinal and urinogenital tracts helps in trapping microbes.

Physiological barriers:

  • Acidity of the stomach kills most ingested microbes.
  • Lysozyme in tears, saliva, and snot kills bacteria by digesting bacterial wall.
  • Pyrogen released by WBC raise body temperature to prevents growth of microbes in out body.
  • Interferon induces antiviral state in non-infected cells.

Phagocytic barrier:

  • Polymorpho-nuclear leukocytes (PMNL-neutrophils), macrophages, and natural killer cells in the blood and tissues kill pathogen by phagocytosis.

Inflammatory barrier:

  • When there is injury to the tissue there is release of histamine and prostaglandins by the mast cells.
  • Due to vasodilation there is leakage of vascular fluid containing serum proteins with antibacterial activity.
  • Further there is influx of Phagocytic cells into the affected area.

Acquired (specific) immunity:

  • It is also known as adaptive immunity.
  • This immunity developed after birth when encountered with pathogen.
  • It supplements the immunity provided by the innate immunity.


  • Acquired immunity has following unique features:
    • Specificity: distinguish specific foreign molecules.
    • Diversity: recognize vast variety of foreign molecules.
    • Discrimination between self and non-self: it is able to recognize and respond to molecules that are foreign or non-self. It will not respond to our own cell or molecules.
    • Memory: after responding to the foreign microbes and elimination, this immune system retains the memory of that encounter (primary immune response). The second encounter with the same microbe evokes a heightened immune response. (Secondary immune response)
  • Acquired immunity is carried out by two special types of lymphocytes:
    • B-lymphocytes.
    • T-lymphocytes.
  • The B-lymphocytes produce a group of proteins in response to pathogen into the blood to fight with them calledantibody.
  • T-lymphocytes do not produce antibody but help B-cells to produce them.

Structure of antibody:

  • Each antibody has four polypeptide chains.
  • Two small chains called light chains.
  • Two longer chains called heavy chains.
  • Antibody represented as H2L2.
  • Different classes of antibody produced in out body are IgA, IgM, IgD, IgE and IgG.

AMI vs. CMI:

  • Immune response by the B-cells by production of antibody is called Antibody mediated immune responseor humoral immune response.
  • Immune response by T-cells is by activation of cytotoxic killer cells which detects and destroys the foreign cells and also cancerous cells called cell mediated immune response.
  • Rejection of organs transplants are due to T-lymphocytes.
  • Tissue matching, blood group matching are essential for organ transplantation.
  • Even after tissue typing immune-suppressants is required before and after transplantation.


Active immunity:

  • When the host is exposed to antigens, which may be in the form of living or dead microbes or other proteins, antibodies are produced in the host body.
  • Active immunity is slow and takes time to give its full effective response.
  • Injecting microbes deliberately during immunization or infection of microbes naturally induce active immunity.

Passive immunity:

  • Ready made antibodies are directly given to protect the body against foreign agents.
  • Colostrums of mother contain abundant antibody (IgA) to protect the child.
  • Foetus receives some antibody (IgG) from mother during pregnancy.

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 protein of pathogen or inactivated/weakened pathogen (vaccine) is introduced into the body.
  • The antibodies produced in the body against vaccine, (antigen) 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.

Passive immunization:

  • Preformed antibody or antitoxin injection for specific antigen.
  • Injection of antivenin for snake bite to counter the snake venom

Vaccine production:

  • Recombinant DNA technology has allowed the production of antigenic polypeptide of pathogen in bacteria and yeast.
  • Vaccine produced by this approach allows large scale production of antigen for immunization. E.g. hepatitis-B produced from yeast.


  • The exaggerated response of the immune system to certain antigens present in the environment is called allergy.
  • The substance to which such immune response is produced is allergen.
  • IgE is produced during allergic reactions.
  • Common allergens are dust, pollen, animal dander etc.
  • Common symptoms are sneezing, watery eyes, running nose etc.
  • Allergy is due to release of histamine and serotonin from the mast cells.
  • Drugs like anti-histamine, adrenalin and steroid quickly reduce symptoms of allergy.

Auto immunity:

  • Memory based acquired immunity able to distinguish foreign molecules or cells (pathogen) from self-cells.
  • Sometimes due to genetic and other unknown reasons the body attacks self cells. This results in damage to the body cells and is called auto-immune disease. E.g. Rheumatoid arthritisMultiple sclerosis.

Immune system in our body:

  • The immune system consists of
    • Lymphoid organs
    • Lymphoid tissues
    • T and B-cells.
    • Antibodies.
  • Immune system recognizes the foreign antigens, responds to them and remembers them.
  • The immune system also plays important role in:
    • Allergic reaction
    • Auto immuno diseases and
    • Organ transplantation.
  • Primary lymphoid organs: bone marrow and thymus, production and maturation of lymphocytes take place.
  • Secondary lymphoid organs: spleen, tonsil, lymph node, Payer’s patches of small intestine and appendix, where proliferation and differentiation of lymphocyte take place.
  • Bone marrow is the main lymphoid organ where all blood cell including lymphocytes are produced.
  • Thymus is a bilobed organ located near the heart, beneath the breastbone.
  • B-lymphocytes are produced and matured in bone marrow.
  • T-lymphocytes are produced in bone marrow but matured in thymus.
  • The spleen
    • Large bean shaped organ mainly contain lymphocytes and phagocytes.
    • Acts as a filter of the blood by trapping blood-borne micro-organisms.
    • Spleen is also serves as the large reservoir of erythrocytes.


  • Lymph node:
    • Small solid structure located at different points along the lymphatic system.
    • Traps the micro-organisms or other foreign antigens.
    • Antigen trapped into the lymph node responsible for activation and differentiation of lymphocytes and cause immune response.
  • Mucosal associated lymphoid tissues (MALT):
    • Located within the lining of major tract (respiratory, digestive and urinogenital tracts)
    • It constitutes 50% of lymphoid tissues.

Leave a Reply

Your email address will not be published. Required fields are marked *

Related Articles


AIDS is a disease where immune system has been too weakened to defend itself against infections. AIDS cannot be treated but prevention is there to avoid AIDS


Uncontrolled cell division results in the formation of tumor, which is of two types benign and malignant tumor. It can be treated with the help of chemotherapy, radiotherapy and alpha interferons.


Feel Free To Email Us .At: