The immune system is responsible for protecting the body against invasion by diseases and other pathogens including viruses, bacteria and parasites. The immune system is also involved in the normal healing response to trauma as a protection against infection. Although the body is an integrated whole and all systems work together to some extent, the major body systems that work with the immune system are the lymph, circulatory, musculoskeletal and digestive systems.
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The thymus is a specialized organ and is part of the immune system. The thymus is located beneath the sternum and is active only in young people. It can be thought of as the immune training ground for the maturing body where immune cells learn to differentiate pathogens from self.
It is through the lymph system that the body moves fluids outside the blood and circulatory system. The lymph is the major pathway for the circulation of immune cells and immune signals. It contains thousands of lymph nodes where immune cells sample and respond to invading pathogens. Tonsils, the appendix and gut-associated lymph tissues (GALT) are similar to lymph nodes and are more specialized organs the body uses to develop and moderate immune responses.
The circulatory system consists of the blood vessels and heart and is a major pathway used by immune cells to travel through the body.
It is through the movement of muscles that lymph is moved throughout the body. This is one of the major reasons regular exercise is critical to healthy immune function. The marrow of the long, flat bones in the body is where blood cells, including the immune cells, grow and mature before entering the circulatory and lymph systems.
The digestive system is critical to proper immune function. The GALT, including the tonsils and appendix, are locations where the body deciphers which foreign substances are food and which are pathogens, helping to create normal immune function.
Along with the four major systems, the respiratory, nervous and endocrine systems are highly involved with immune function. The lungs are integrally involved with exposure of the immune system to pathogens and, like the digestive system, have large amounts of immune tissues. The nervous and endocrine systems share many of the same signaling chemicals as the immune system and along with the immune system are essential to the body's balance and homeostasis (ability to maintain internal equilibrium).