INSIGHT-AM is an MRC-funded study investigating the use of an inhaled intervention on the function of lung immune cells called Alveolar Macrophages. We are using an intervention called recombinant human granulocyte-macrophage colony stimulating factor (rhuGM-CSF), and are studying its effects in different groups of people. These groups include healthy young and healthy old participants, as well as critically ill individuals on Intensive Care Units (ICUs). We aim to learn more about the function of Alveolar Macrophages, and the impact of rhuGM-CSF administration. Our hope is that by determining the mechanisms by which rhuGM-CSF can enhance the function of AMs, we can proceed to further trials aimed at preventing infections such as pneumonia in the most vulnerable populations, and contributing to the vital study of finding non-antibiotic-based treatment strategies for severe infections.