This article reviews the development of the immune system from infancy to old age, highlighting its maturation and decline. At birth, the immune system is immature, with the innate immune system providing early defense and the adaptive immune system developing in the thymus. Infants are at risk from various pathogens but have a good chance of survival due to improved nutrition, hygiene, and vaccination. The immune system gradually matures during childhood, with critical protection against infectious diseases. In adulthood, the immune system acquires memory and can respond to new antigens, but it also faces challenges such as cancer and autoimmunity. In old age, the immune system declines, leading to increased susceptibility to infections, chronic inflammation, and autoimmune diseases. The article discusses the role of the immune system in pregnancy, malignancy, and autoimmunity, and explores the evolutionary pressures that have shaped the immune system's development.This article reviews the development of the immune system from infancy to old age, highlighting its maturation and decline. At birth, the immune system is immature, with the innate immune system providing early defense and the adaptive immune system developing in the thymus. Infants are at risk from various pathogens but have a good chance of survival due to improved nutrition, hygiene, and vaccination. The immune system gradually matures during childhood, with critical protection against infectious diseases. In adulthood, the immune system acquires memory and can respond to new antigens, but it also faces challenges such as cancer and autoimmunity. In old age, the immune system declines, leading to increased susceptibility to infections, chronic inflammation, and autoimmune diseases. The article discusses the role of the immune system in pregnancy, malignancy, and autoimmunity, and explores the evolutionary pressures that have shaped the immune system's development.