Complex sentence: Once in the lungs, bacteria may invade the spaces between cells and between alveoli, where the macrophages and neutrophils (defensive white blood cells) attempt to inactivate the bacteria.
Split into simpler sentences: Once in the lungs, bacteria may invade the spaces between cells and between alveoli. In the lungs, alveolar macrophages and neutrophils, types of defensive white blood cells, attempt to inactivate the bacteria.