The main characteristic of an aging immune system compared to a younger one is that the elderly produce fewer cells needed to fight disease. These include killer cells and white blood cells, both of which are needed to fight disease. The cells that remain can become slower and slower to respond to disease. This leads to older people often having compromised immunities, making them more susceptible to certain diseases.
Older people have the same immune functions as younger people, but theirs work a bit more slowly. The exact amount your immunities lower will depend on many factors, including a person's diet, exercise levels, and how strong their immune system was to begin with. Those who live healthy lifestyles in their youth are more likely to have stronger immune systems when they are older.
The human immune system is made up of many cells. White blood cells find invaders in the body and alert them to the production of antibodies. These are markers that attach to viruses and bacteria to let additional cells, called killer cells, know to destroy themselves. White blood cells also begin to replicate rapidly when an intruder is detected so that they can be alerted and create more antibodies and killer cells. This is why the white blood cell count is unusually high in those who have an infection.
In most individuals, the characteristics of an aging immune system involve slow cell production. This leads to a lowered immune response as there are not as many white blood cells around to alert the production of antibodies. Because of this, the immune system responds more slowly to infections. In some cases, this allows infections or illnesses that are not serious in young adults to become more serious in the elderly. For example, they are at much higher risk of complications and hospitalization due to the flu.
There are some ways to compensate for the disadvantages of the aging immune system. Some vaccines have been shown to increase the number of white blood cells in older people. Others are designed to help the elderly build antibodies, even if they build more slowly, against common illnesses. This gives them a head start in building strong immunities should they come into contact with the real virus.
As with the young, older people can often reverse the effects of aging on the immune system by eating a healthy diet and getting plenty of exercise. Although they may not be able to return their immunities to their former strength, they can help their bodies fight infections more effectively. This helps prevent serious complications in case of illness.