Even though the development of the immune system begins from the fifth week of the embryonic period, the system is still under development at the birth time. For example, the phagocyte cells functionally develop during the first 6 months of neonatal life. Also, A few mature and effective T cells, as the commander of the immune system, circulate in the tissues, B cells have poor efficiency as antibody-producing cells, and some antibody subclasses are not synthesized up to two years of age. Furthermore, the development and formation of some B cells in marginal zone of the spleen last up to two years of age. These cells play a major role in antibody responses and primary defense against pathogens, particularly those agents entering the body via the blood. On the other hand, the neonate is exposed to many pathogens and infectious diseases until the final formation of the immune system. Breast milk, in addition to its nutritional value, plays an important role in the protection of neonate against microorganisms and contains innate immunity factors including probiotics and antimicrobial peptides, as well as acquired immunity factors including leucocytes and the IgA class antibody, thereby granting the neonate immunity against infectious factors. In the Holy Qur'ā, n, seven verses discuss about the breast milk. In the most well-known verse about breast milk (Verse 233 of Sura al-Baqarah), the phrase "Hawlaini Kā, milaini (in Arabic: ِنیَلوَح ِنیَلِماک)" refers to the full breastfeeding period (maximum useful period) i. e., two whole years. Due to the underdeveloped immune system and the absence of efficient defensive cells during the first two years of neonates’,life, they seem to be susceptible to infectious diseases up to this period. Therefore, breastfeeding, by transferring the mother's immune factors to the neonate, supports the neonate's immune system against infectious agents and diseases. In conclusion, perhaps one of the reasons that the Holy Qur'ā, n recommends complete breastfeeding is due to the neonates’,underdeveloped immune system in this period.