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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 bodys 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 bodys own healthy tissues alone Sometimes however the immune system attacks and damages these healthy tissues This reaction is called an autoimmune response or autoimmunity The immune system is composed of many parts that work together to fight infections when pathogens or poisons invade the human body Pathogens are disease- causing organisms such as bacteria and viruses The immune system reacts to foreign substances through a series of steps know as the immune response Any agent perceived as foreign by a bodys immune system is called an antigen Several types of cells may be involved in the immune response to antigens When an antigen enters the body it may be partly neutralized by components of the innate immune system It may be attacked by phagocytes or by performed antibodies that act together with the complement system The human immune system contains approximately 1 trillion T cells and 1 trillion B cells located in the lymphoid organs and in the blood plus approximately 10 billion antigen-presenting cells located in the lymphoid organs 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
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