Your immune system is a complex network of tissues and cells that keeps you healthy by detecting threats to your body and eliminating them. The immune system works to eliminate both cells that are damaged by accident or injury, and foreign bacteria and viruses invade your body and cause disease and infection. For example, the redness and blistering of a sunburn is an inflammatory response of neighboring healthy cells working to destroy skin cells damaged by UV rays.
Your immune system is located in many places in your body. Your bone marrow is continually generating immune cells which mature into specialized cells in different parts of the body, including the thymus and spleen. Immune cells circulate in your bloodstream and your lymphatic system. They are concentrated in your skin, and on the mucus-lined surfaces of your respiratory and digestive systems where your body makes first contact with many pathogens (germs, viruses and parasites).
When the immune cells detect danger to the body, they respond by multiplying and mounting an attack on the specific threat. If immune cells are not able to completely destroy the pathogen, you become sick or develop an infection. Once your body has overcome the infection, "memory cells" that remember how to attack that particular pathogen keep circulating in your body, ready to destroy it immediately if it appears again.
You are probably already immune to many of the bacteria circulating in your local community. However, you might become sick when you travel to another country, or when your children start school and are exposed to unfamiliar germs.
Pregnant women and people with genetic immune disorders or immune-suppressing infections like HIV are vulnerable to germs that typically do not cause infection in healthy individuals. Studies have found that our immune system weakens as we age. If your immune system is compromised, avoid situations where you might be exposed to disease or infection and follow your doctor's recommendations.
The immune system is so complex and is affected by so many factors that scientists have not been able to prove that any particular nutrient or behavior can "boost" or strengthen your body's defense against disease. Each person is genetically different. While your aunt might swear that taking extra zinc or vitamin C helps her get over a cold in one day, you might experience no benefit at all. While a good multi-vitamin can help by making up for deficiencies in your diet, taking too much of anything can be harmful.
The conclusion is that your immune system, like the rest of your body, functions best when you follow a healthy lifestyle:
Studies have indicated that stress and traumatic experiences affect the immune system. Good social relationships and a positive attitude contribute to a strong immune system and your body's ability to overcome disease.
How to Boost Your Immune System (www.health.harvard.edu/staying-healthy/how-to-boost-your-immune-system), Harvard Health Publications
Overview of the Immune System (http://www.niaid.nih.gov/topics/immuneSystem/Pages/overview.aspx), National Institute of Allergy and Infectious Diseases