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:

• Readymade 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.

Allergies:

• 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 This results in damage to the body cells and is called auto-immune disease. E.g. Rheumatoid arthritis, Multiple 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.