Lung macrophages begin to develop in humans only after birth when the lungs are first inflated with inhaled air, reveals a new study. The researchers at Karolinska Institutet, Sweden, who were able to directly study the development of human macrophages in a living lung, found that human lungs are exposed to microorganisms, such as bacteria and viruses, from our first breath. 

“We are protected from most infections at an early age by certain immune cells in the lungs, so-called macrophages, developed after its exposure to microorganisms,” the researchers wrote in their report. The research, published in the Journal of Experimental Medicine, showed how lung macrophages develop in a living lung and revealed that the lung macrophages develop in two different ways; In the first type of development, lung macrophages originate from precursor cells that are already present in the fetus’ liver, and these precursor cells move from the liver to the lungs via the bloodstream after we are born. In the lungs, they are then exposed to various growth factors, which helps them to develop into ‘mature’ lung macrophages. The second type of development occurs later in life and at that point they develop from adult precursor cells, so-called monocytes, which are found in the blood. The new findings are expected to be significant for the continued development of important lung disease treatments.