Question

Write an essay on the immune system of human body.
(3-4 pages)
please answer it fast

Answer 1

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The Immune System

     The immune system is a group of cells, molecules, and tissues that help defend the body against diseases and other harmful invaders. The immune system provides protection against a variety of potentially damaging substances that can invade the body. These substances include disease-causing organisms, such as bacteria, fungi, parasites, and viruses. The body's ability to resist these invaders is called immunity. A key feature of the immune system is its ability to destroy foreign invaders while leaving the body's own healthy tissues alone.
Sometimes, however, the immune system attacks and damages these healthy tissues.
This reaction is called an autoimmune response or autoimmunity. 
To maximize the chances of encountering antigens wherever they may invade the body, lymphocytes continually circulate between the blood and certain lymphoid tissues. A lymphocyte spends an average of 30 minutes per day in the blood and recirculates about 50 times per day between the blood and lymphoid tissues.
Lymphocytes are special types of white blood cells. Like other white blood cells, lymphocytes originate in the bone marrow, the blood-forming tissue in the center of many bones. Some lymphocytes mature in the bone marrow and become B lymphocytes, also know as B cells. The B stands for bone marrow derived. Some of these cells develop into plasma cells, which produce antibodies. Antibodies are proteins that attack antigens. They are carried in the blood, in tears, and in secretions of the nose and the intestines. Other lymphocytes do not mature in the bone marrow. Instead, they travel through the bloodstream to the thymus, an organ in the upper chest. In the thymus, the immature lymphocytes develop into T lymphocytes, also knows as T cells. The T stands for thymus derived. The B lymphocytes are responsible for the production of the blood-serum components called immunoglobulins.