Alveolar macrophages in the lungs are part of the innate immune system and help prevent infections by removing microorganisms and clearing toxic substances. They produce substances that trigger an inflammatory response and can also resolve inflammation. During infections, they clear dead neutrophils to prevent harm. In pneumococcal pneumonia, they play a role in both the inflammatory and anti-inflammatory response, preventing lung damage.
Alveolar macrophages are cells found in the lungs that are part of the immune system. Macrophages are phagocytes, which means they are capable of absorbing and digesting dead or damaged cells and potentially harmful substances such as bacteria or toxic particles. Unlike other macrophages, they are directly exposed to the environment outside the body due to their location on the lining of the lungs. There, they help prevent infections by removing microorganisms before they enter the respiratory circulation. They also clear allergic and toxic substances from the air spaces in the lungs.
Human alveolar macrophages are the main phagocytes in what is called the innate immune system, which is the body’s first line of defense against harmful microbes and toxic substances. Alveolar macrophages are capable of dealing with the small number of bacteria and viruses that are commonly breathed in each day. They engulf and destroy microorganisms, and also secrete substances that attack microbes. If more microorganisms invade the air spaces, alveolar macrophages are able to produce substances that trigger an inflammatory response, attracting more immune cells into the lungs.
In addition to initiating inflammation to fight disease, the human alveolar macrophage is also involved in resolving that inflammation. An alveolar macrophage cell line can produce different types of alveolar macrophages, referred to as M1 and M2 cells, which are thought to have opposite functions. M1 is the cell that is most affected by inflammation, while M2 has more of an anti-inflammatory effect. Different situations can lead to the production of more or fewer of each cell type.
During an infection, the inflammatory response leads to white blood cells known as neutrophils dying and collecting within the lung air spaces. The cellular contents of these neutrophils could be harmful if they leak out. As part of their anti-inflammatory function, alveolar macrophages are able to clear dead neutrophils before they break down and release their contents into the lungs.
When people develop pneumococcal pneumonia, both the inflammatory and anti-inflammatory actions of alveolar macrophages come into play. A massive inflammatory response causes the air spaces of the lungs to fill with fluid and neutrophils, but, through the action of alveolar macrophages, these spaces clear again without lung damage. Pneumococcal pneumonia can be treated with antibiotics or prevented with a vaccine.
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