The human immune system consists of specialized cells that work together to protect the body from bacteria, viruses, parasites and fungi, and also fight the growth of tumor cells. Many important organs are in the immune system. The bone marrow produces the cells of the immune system, including the T cells, B cells, natural killer cells, macrophages, leukocytes and dendritic cells. The thymus is where T cells mature. The spleen filters the blood and captures the foreign invaders while the lymph nodes filter the lymph fluid. Although extremely important to good health, the immune system accounts for only 1 percent of the body's cells, according to the Mansfield Connecticut School System.
White Cell Production
White blood cells are responsible for providing the body with immunity. Different types of white blood cells work together to recognize the foreign invaders, attack them, suppress themselves once the invader is eliminated and create a memory for recognition at the next encounter. With more than 1 trillion white blood cells in the body that live for only a few days to a month, the bone marrow produces 100 billion new cells a day, according to the Merck Manual.
Stress Depresses the Immune System
The body is equipped with the ability to handle stress. The body reacts to all types of stress in the same way, initiating the fight-or-flight response. When presented with a perceived threat, the hypothalamus in the brain sends signals to the adrenal glands near the kidneys to produce and release more of the hormones adrenaline and cortisol. These hormones prepare the body to deal with stress by increasing the heart rate, blood pressure and level of sugar (used for energy) in the blood.
Cortisol depresses the functions of the body that are non-essential to the fight-or-flight response, according to the Mayo Clinic. This includes suppressing the immune system. Continually living with stress leaves the fight-or-flight mechanism turned on, which leaves the body wide open to foreign invaders.
Exercise, Immunity and Moderation
Participating daily in moderate exercise helps the immune system fight off infections while also decreasing illnesses such as heart disease and osteoporosis, according to the National Institutes of Health. However, those who take part in vigorous workout routines might be decreasing their number of white blood cells and increasing the levels of cortisol, which further suppresses the immune system.
Dieting Affects the Immune System
A study conducted by the Fred Hutchinson Cancer Research Center in Seattle found that repeated dieting, often referred to as yo-yo dieting, negatively affects the immune system. The study measured the number of natural killer cells, which are the immune cells that effectively kill viruses and cancer cells, and found that those who had frequent weight loss episodes had fewer natural killer cells than those who maintained a constant weight over time. The solution is to lose the weight, but work to keep it off.
