The circulatory system and immune system serve the same, yet complementary purpose, to keep the entire body alive. The circulatory system does this by delivering oxygen from the lungs to every organ and cell in the body. The immune system does this by consuming foreign substances before they can damage the body. The connection between the circulatory system and immune system lies in the fact that the immune system travels through the entire body, inside the circulatory system.

The circulatory system has three major components: the heart, the lungs, and blood vessels. The lungs absorb oxygen from the air and creates a form of it that passes into the blood stream. The blood vessels then carry this blood through every organ in the body. This is all powered by the heart's contractions, which provide the force behind the blood's circulation through the entire body.

Large blood vessels connect the different organ systems. These large vessels split off into a number of smaller vessels, however, that pass through all the tissue in any given organ system. This allows the blood to deliver the lung-created oxygen to all the cells in the body.

Because the various specialized cells that compose the immune system travel in the blood stream, and the circulatory system carries this blood throughout the body, the immune system has access to every part of it. This allows lymphocytes, the scouts of the immune system, to look for foreign bodies in every organ system. When these lymphocytes detect a potentially harmful substance, they secrete special markers called antibodies that attach to the foreign invaders.

These markers attract the attention of another type of immune system cell that travels in the circulatory system, called macrophages. These cells act as the soldiers of the immune system, attacking and consuming the marked foreign substance in order to neutralize it. Lymphocytes will continue to release antibodies into the blood stream to attract more macrophages until the foreign substance has been completely consumed.

The connection between the circulatory system and immune system is usually beneficial, but can prove harmful if an individual develops an autoimmune disorder. These are conditions in which the immune system can no longer tell the difference between the body's own cells and foreign substances. This can lead to the immune system attacking and destroying the tissue from any organ system, including the very blood vessels through which the immune system travels.