Figure 1.

Environment surrounded by microbial components is effective only during early childhood. Only T_n's are present during infancy. The T_h2-dominant immune system will develop when the immune system is exposed to allergens without microbial components such as endotoxin. Allergen-associated T_h2 adjuvants such as prostaglandins may enhance T_h2 development. On the other hand, antigen/allergen-specific T_h1 cells can develop by simultaneous exposure to antigen/allergen and microbial components. After childhood, microbial exposure cannot drastically alter the proportion of T_h1/T_h2 balance because the proportion of T_n is decreased. T_n indicates naive helper T cells.