How does the immune system work together with the circulatory system to function properly?
a. chemicals released by immune cells cause increased blood flow to an infected area; this increased blood flow brings other immune cells to the area more quickly.
b. red blood cells move through the body (to parts where they are needed) in the blood vessels within the circulatory system to fight antigens.
c. oxygen-depleted blood is delivered to an infection site by the circulatory system's arteries, so the immune system can work harder.
d. the immune system's pathogens work to fight and destroy invaders.

The correct answer is A.

When a tissue of the body has been injured, the immune system of the body kicks into action immediately. The body produces an inflammatory response that is localized around the site of injury. The inflammatory response is a defense mechanism that protects organisms from infection and injury.

During an inflammatory response, the small blood vessels dilate (expand their lumen), a process called vasodilation, which causes increase of blood flow  into the area.  White blood cells and special serum proteins are speedily brought by the blood to the site of injury to fight any bacteria and foreign bodies that may have gained access to the body  through the broken tissue.

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