Human Defense System

• Immunity is the ability to resist infection.
• The general defense system is non-specific and acts against all pathogens. The skin and mucous prevent entry of pathogens and the white blood cells destroy any pathogens.
• Phagocytic white blood cells engulf and destroy harmful bacteria and viruses. ​Some phagocytes are large and are known as macrophages. Some macrophages move around the body in the fluid ​while others stay in the spleen and​ lymph nodes.
• Defense proteins are 20 proteins that are found in the plasma. They are activated by infections which destroy​ harmful pathogens. Interferons are another set of defense​ proteins. 
• Infected cells release a chemical that causes blood capillaries to become more porous. ​Causes localized swelling, redness, heat and pain which brings more white blood cells to the area of infection. 
• The specific defense system attacks particular pathogens. It functions by production of antibodies or when white blood cells destroy body cells that are infected by a pathogen. 
• Monocytes develop into white blood cells called macrophage. ​​They recognize foreign molecules that are found on the surface of pathogens. ​​When a macrophage digests a pathogen, antigens from the pathogen then stimulate the production of antibodies. ​
• An antigen is a foreign molecule that stimulates the production of antibodies. e.g. cell walls of bacteria
• An antibody is a protein produced by white blood cells in response to an antigen. 
• Antibodies prevent viruses and bacteria from entering new host cells and are then destroyed by phagocytes.
• After infection, some of the antibody producing lymphocytes remain in the body for a long time. ​​These memory lymphocytes respond rapidly if a similar antigen re-enters the body later by producing specific antibodies. 
• Induced immunity is the ability to resist diseases caused by specific pathogens by the production of antibodies. 
• Active immunity involves the production of a person’s own antibodies in response to antigens that enter the body and is ​​long lasting.
• Natural active immunity occurs when a pathogen enters the body in the normal way. 
• Artificial immunity occurs when a pathogen is medically introduced into the body. e.g. vaccination
• A vaccine is a non disease causing dose of a pathogen, which triggers the production of antibodies.
• Passive immunity occurs when individuals are given antibodies that are formed by another organism. It p​rovides short term resistance to infections. 
• Natural passive immunity occurs when a child gets antibodies from its mother. Artificial passive immunity occurs when a person is given an injection containing antibodies made by another organism.
• B-lymphocytes (B-cells) mature in Bone marrow​ and T-lymphocytes (T-cells) mature in the Thymus gland​.

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• Each B-cell can only recognize one type of antigen.​ Each B-cell only produces one type of antibody. ​Antibodies inactivate antigens by attaching to them.​ Phagocytes then dispose of the cell with the antigen.
• T-cells do not produce antibodies but instead destroy viruses and some bacteria by ​Helper T-Cells,​ Killer T-cells,​ Suppressor T-Cells,​ and Memory T-cells.
• Helper T-cells recognize antigens and stimulate the production of B-cells. Killer T-cells attack cells containing a foreign antigen. Suppressor T-cells suppress the immune response and memory T-cells memorize the antigen.