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INTRODUCTION

Immunology is the branch of biology and medical science that focuses on the study of the immune system, which is responsible for defending the body against foreign substances, pathogens, and abnormal cells

The immune system plays a crucial role in maintaining the overall health and well-being of an organism .Animals  and even plants have several defenses against foreign substances, pathogens, and abnormal cells.  The defenses keep your body’s environment healthy most of the time.,There many lines of defence available for animals , namely:

First-Line Defenses– The body’s first line of defense against pathogens uses mostly physical and chemical barriers such as sweat, skin, tears, mucus, stomach acid, and so on.  Our skin and other membranes which line the body passages are fairly effective in keeping most pathogens out of the body.  Mucus can trap pathogens, which are then washed away or destroyed by chemicals.  Tears, sweat, and saliva have certain chemicals which can kill different pathogens.

Second-Line Defenses– If a pathogen is able to get past the body’s first line of defense, and an infection starts, the body can rely on it’s second line of defense.  This will result in what is called an inflammatory response.  This is a reaction that causes redness, heat, swelling, and pain in the area of infection.   Redness and heat are due to capillary dilation resulting in increased blood flow.  Swelling is caused by the passage of plasma from the blood stream and into the damaged tissue.  The pain is mainly due to the tissue destruction, and to a lesser extent, the swelling.

Third-Line Defenses – Sometimes the second line of defense is still not enough and the pathogen is then heading for the body’s last line of defense, the immune system.  The immune system will recognize, attack, destroy, and remembers each foreign substance and pathogen that enters the body.  It does this by making specialized cells and antibodies that makes the pathogens useless.  Unlike the first line and second line defense the immune system determines between kinds of pathogens. For each type of pathogen, the immune system produces cells that are specific for that particular pathogen. 

The Immune Response – The immune system includes all parts of the body that help in the recognition and destruction of foreign materials.  White blood cells, phagocytes and lymphocytes, bone marrow, lymph nodes, tonsils, thymus, and your spleen are all part of the immune system. immunity is the body’s ability to fight off foreign substances, viruses, or bacteria by producing antibodies or cells that can deactivate these foreign substances or cells.  The whole idea of immunity is that the body is able to distinguish between its own substances or foreign substances.  When the body recognizes foreign cells or molecules, it makes special antibodies or cells that attach to the pathogens and inactivate them.  When these antibodies and special cells are produced it is called the immune response.  .