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Understanding Immune System: Natural and Acquired Immunity

Learn about the body's defense mechanism against pathogens through Natural (innate) and Acquired (specific) immunity. Discover the roles of T and B lymphocytes, antibodies, and the differences between cellular and humoral immunity.

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Understanding Immune System: Natural and Acquired Immunity

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  1. Immunity Is the resistance of the body against pathogenic agents Types-- Natural (innate) immunity and Acquired immunity

  2. Natural/ innate/ nonspecific immunity • It is present since birth, prevents entry of micro organisms into the body. • It is the first line of defense against any pathogens. • Mechanisms involved— • Phagocytic activity of WBCs • Acid in the stomach and the enzymes in GIT • Lysozymes present in the tears and saliva • Protective function of skin

  3. Acquired immunity /specific immunity If the invaders overcome the natural immune system, then the acquired immune system comes into play. It is the resistance developed against any specific foreign body, so it is also known as specific immunity. It’s the most powerful immune mechanism Types– Cellular immunity by T lymphocytes and Humoral immunity by B lymphocytes.

  4. Development of acquired immune system During the fetal development and neonatal life, some lymphocyte precursors from the bone marrow enters the thymus gland and becomes transformed into T-lymphocytes and some precursors enter liver, spleen and bone marrow and get transformed into B-lymphocytes

  5. Schematic chart to show the role of T and B lymphocytes in immunity Suppressor T cell

  6. Thymosin a hormone secreted by thymus accelerates the proliferation and activation of lymphocytes in thymus. Types—Helper T cell or inducer T cell Cytotoxic T cell or Killer Tcell Suppressor T cell Memory T cell After transformation the T lymphocytes migrate and stay in lymphoid tissues present in lymph nodes, spleen, bone marrow and GIT .

  7. B-Lymphocytes were first discovered in “bursa fabricius” in birds They are transformed into two types Plasma cells and Memory cells After transformation they migrate and stay in lymphoid tissue. Plasma cells forms and secrete protein Immunoglobulins IgG, IgA, IgM, IgD and IgE

  8. Development of cell mediated immunity It develops when an antigen from the invading micro organism is exposed to the T-lymphocytes. The exposure or presentation of antigen to the lymphocytes is done by-Macrophages B lymphocytes and Dendritic cells The antigenic products activate the helper T cells and B-lymphocytes The macrophages also secrete interleukin-1, which brings about activation and proliferation of lymphocytes

  9. Helper T cell— Promotes various activities of immune system; it stimulates other T cells and B cells. Two types of helper T cells- TH1—secretes IL2 and gamma-interferon concerned with cellular immunity. TH2—secretes IL4 and IL5 concerned with humoral immunity.

  10. Cytotoxic T cell— The activated T cells comes into circulation to destroy the invading organism Mechanism of action– The membrane of cytotoxic T cells contain receptor protein to which the antigens or organisms tightly bind with. The organism is destroyed due to release of cytotoxic substances from the T cell. The cytotoxic T cells can also destroy— Cancer cells, transplanted cells tissues affected by foreign bodies.

  11. Suppressor T cells— Suppresses the activities of cytotoxic T cells and prevents it from destroying the body’s own tissue. It also suppresses helper T cells. Memory T cell— Some of the activated T cells do not enter the circulation but migrate to various lymphoid tissue, and get activated when body is exposed to the same antigen again. The response is immediate and more powerful. Specificity of T cells—Each T cell gets activated by only one type of antigen.

  12. Development of humoral immunity • Is the major defensive mechanism against bacterial infection • The antigen of the invading organism is presented to B lymphocytes to activate them and also helper T cells. • The B lymphocytes proliferate and transform into memory B cells and “plasma cells”. • The plasma cells secrete large quantity of antibodies (about 2000 molecules per second) • Antibodies are produced till the end of lifespan of plasma cell (several days to weeks).

  13. Memory B cells The memory B cells remain inactive until the body is exposed to same antigen for the second time. The antibodies produced on first exposure is less in concentration, takes longer time for production and belong to IgM type. On second exposure, the antibody produced is faster, more in concentration and belongs to IgG type. This forms the basic principle of vaccination against infection.

  14. Graph showing primary and secondary immune responses

  15. Cellular and humoral immunity differences

  16. Cellular and humoral immunity differences continued

  17. Antibodies / Immunoglobulins • Produced by B lymphocytes, are gamma globulin fraction of plasma, form 20% of plasma proteins.

  18. Structure of immunoglobulin

  19. Mechanism of action of antibodies— Direct action– agglutination precipitation neutralization lysis

  20. Action of antibody through complement system This action is stronger than direct action, as the complement system accelerates the fight against invading organism. Complement system involves plasma enzymes named C1 to C9. C1 has 3 subunits C1q, C1r, C1s. These enzymes are in inactive form and get activated by 2 ways---Classical pathway and Alternate pathway.

  21. Classical pathway— C1 binds with the antibodies and triggers a series of events to activate other enzymes. The enzymes produce following activities— Opsonization Lysis Chemotaxis Agglutination Neutralization Activation of basophils.

  22. Alternate pathway— Activation of alternate pathway is triggered due to binding of a protein present in the circulation “factor1” with the polysaccharide present on the cell membrane of invading organism. This activates C3 and C5 which ultimately attack the organism.

  23. Immunization Is done by subjecting the individual to an antigen to induce the immune resistance of the body, to fight against a specific disease. Passive immunity– No challenging of immune system of the body. Done by administration of serum or gamma globulins from a person who is already immunized Passive natural– maternal antibodies Passive artificial– injecting antibodies from other sources

  24. Active immunity—immunization is acquired by activating the immune system. Active natural—infection Active artificial—vaccines, toxoids

  25. Types of immunity

  26. Immune deficiency diseases— Due to defective component of immune system Types—Congenital immune deficiency disease eg-Di George’s syndrome Acquired immune deficiency disease (AIDS) Autoimmune diseases— IDDM Myasthenia gravis Rheumatoid arthritis

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