ABSTRACT
The prevalence of atopic disorders or allergic diseases such as eczema, allergic rhinitis and asthma has rapidly increased worldwide in the past several decades, especially in the industrialized countries. One explanation for the increasing prevalence of allergic diseases is the “hygiene hypothesis”, which postulates that decreased exposure to immunostimulating pathogens in early childhood causes increased prevalence of allergic diseases in later age (Strachan 1989). The hygiene hypothesis can be interpreted as follows: A highly sanitized environment provides insuffi cient bacterial stimulation to the neonate. The immune balance in the neonate leans toward type 2 T helper (Th2) cell immunity, and insuffi cient bacterial stimulation causes a lack of shifting of immune function from a Th2 to a Th1 profi le and reduced activation of regulatory T (Treg) cells. Treg can suppress immune responses and is essential for induction of peripheral tolerance to both self and foreign antigens (Romagnani 2004; Kaisho and Akira 2006). Toll-like receptors (TLRs) have been identifi ed as molecular structures present on cells involved in innate immunity, which interact with components of bacteria and viruses. It has been demonstrated that bacterial components
affect the differentiation and development of T-cells via TLRs (Kaisho and Akira 2006), and thus the hygiene hypothesis has achieved general acceptance immunologically.
