Homeostasis is the process in which the body maintains normal, healthy ranges for things like temperature, energy intake and growth. The immune response contributes to homeostasis by preparing the body to fight off infection and to help the healing process in case harm occurs. During infection, the immune system will cause the body to develop a fever. The immune system also causes an increase in blood flow to bring oxygen and other immune cells to sites of infection. In addition, the immune system helps in wound healing, so that proper barriers in organs can be reformed such that those organs can correctly participate in homeostasis.
When a body is infected by bacteria or viruses, the body must invest a lot of energy to fight off the invaders. Otherwise, there is no point in maintaining proper homeostasis of water levels and such if the whole organism is going to die from infection. Thus, the brain is able to detect molecules called pyrogens, which are released by infected cells or infectious agents. Pyrogens tell the brain to increase body temperature by telling the body to retain heat. This results in a fever. Fevers are meant to slow down bacteria and viruses, which do not like high temperatures. This buys more time for the immune cells to find and eliminate the invaders.
Increased Blood Flow
A site of injury or infection will turn red, swell, feel warm and feel tender. These are the symptoms of what is called inflammation. Immune cells rush to the site of damage and release chemicals that cause these symptoms. In particular, mast cells are immune cells that release chemicals that cause blood vessels at the site of injury, which can be a bruise or a cut, to enlarge. This enlargement brings more blood to the site of injury, which brings more oxygen to sustain the burst of repair activity and also brings more immune cells to the rescue. More blood flow means faster repair. Faster repair means the body can get back to normal faster.
Wound healing is the process in which a damaged tissue is repaired. Damage can be a cut in the skin or a bruise in a muscle. At the site of damage, dead or broken cells are eaten by immune cells called macrophages. In damaged skeletal muscle, macrophages accumulate at the site of injury and release a protein that causes muscle cells to regrow. In damaged skin, macrophages fill up the wound and release chemicals that cause new blood vessels to form. These blood vessels will be necessary to bring nutrients to and remove wastes from the new skin cells that will form. Until the wound is repaired, the body is at higher risk of infection and homeostasis cannot be fully attained.
Immune cells called T or B lymphocytes become activated for battle after they encounter foreign proteins that were captured from invading organisms. After finding a protein molecule from a particular type of foreign invader, T and B cells train themselves to fight against this invader. T and B cells can undergo what is called clonal selection, which is the process in which they divide to make two different types of copies of themselves. One type of copied cell is called the effector cells, which go right into battle fighting invaders. The other type of copied cell is called memory cells, which stay inactive in the body for a long time, waiting to encounter the same invader in the future so that they can mount a faster attack the second time around. Memory cells make the body better prepared for future invasions, which makes it easier to maintain homeostasis in the future.