4.How do environmental factors affect to our immune system?

Some environmental factors such as smoking and dust affect the susceptibility for allergic disease. In contrast, there are interesting reports implicating that contact with bacteria protect from allergic disease. This hypothesis is called a hygiene hypothesis. For example, allergic disease has been increased in developed countries using antibiotics and clean products. Furthermore, it is reported that the children growing up in farming area where they have more chance to exposure to bacteria are more resistant to asthma development compared with urban area. These suggest that that exposure to bacteria protects from allergic disease development, though there are still no define evidence.

The allergic diseases are caused by inappropriate immunological responses to harmless antigens called allergens. The mechanism of hygiene hypothesis is unclear, but it is proposed that bacteria induce the immune response to stimulate immune cells eliminating bacteria and down-regulates to produce allergic antibody for harmless antigens. In other words, the insufficient stimulation of anti-bacterial immune response leads to an overactive response against allergens, which in turn led to allergic disease. The children living in too sterile an environment are not exposed to enough bacteria to keep the immune system busy.