Environment surrounded by microbial components is effective only during early childhood. Only Tn's are present during infancy. The Th2-dominant immune system will develop when the immune system is exposed to allergens without microbial components such as endotoxin. Allergen-associated Th2 adjuvants such as prostaglandins may enhance Th2 development. On the other hand, antigen/allergen-specific Th1 cells can develop by simultaneous exposure to antigen/allergen and microbial components. After childhood, microbial exposure cannot drastically alter the proportion of Th1/Th2 balance because the proportion of Tn is decreased. Tn indicates naive helper T cells.