Cells of the Lower Airways

Horses normally have a scant amount of mucus in their lower airways, along with alveolar macrophages, lymphocytes and a scant number of neutrophils, mast cells, and eosinophil.

Alveolar macrophages are the predominant cell in the airways of most mammals, including humans, but they only account for approximately 60% of the total cell population in horses; lymphocytes are the other heavy hitters, accounting for about 40% of the total cell population in the lungs. Normal horses have less than 5% neutrophils, less than 2% mast cells, and less than 0.5 % eosinophils. Let’s talk for a minute about what these cells do.


We think of alveolar macrophages (AMs)as the normal cells of the lung.

In this image, we see a dark blue lymphocyte, a macrophage with large lacy cytoplasm, and a mast cell (poorly staining without toluidine blue)

However, they are not just there for the ride – AMs play a critical role in the immune response of the lung. On BALF, they are most recognizably ‘clean-up’ cells – that recognize particulates or infectious invaders such as bacteria that don’t belong in the lung and get rid of those particulates by ingesting them. [PIC of alternaria spore in AM] Alveolar macrophages, interestingly, actually play a very important role in down-regulating the daily immune response of the lung. If the lungs responded vigorously to every foreign particle that gets introduced to the lower airways, the lungs would be in a constant state of inflammation – which would not be useful for performance! The proper function of AMs in producing anti-inflammatory cytokines on a regular basis and switching to produce inflammatory cytokines when they are needed is critical to lung homeostasis. This is why, when we investigate a BALF cytology, we don’t just report the numbers of AMs – we also report whether they have engulfed particulates or whether they have formed giant cells or are cleaning up portions of red blood cells called hemosiderin.

This BAL cytology came from a horse that had raced 2 weeks ago and had gone at speed on the treadmill the same day as the BAL was performed. Hemosiderophages lie in a bed of red cells.


After the AMs, horses have small and large lymphocytes in the BALF. Lymphocytes are critical to the proper function of the lung’s immune system, and can produce many of the inflammatory cytokines that are involved in both IAD and heaves.