Histology, Alveolar Macrophages

Excerpt

Alveolar macrophages, also known as dust cells, are phagocytic cells that play a crucial role in the immune defense of the respiratory system (see Image. Alveolar Macrophage). As part of the innate immune system, alveolar macrophages serve as the first line of defense against inhaled pathogens and particulate matter in the pulmonary alveoli and interalveolar septae near pneumocytes.

The alveoli, the terminal units of the respiratory system, are responsible for gas exchange between the lungs and the bloodstream. The alveolar structure consists of 3 primary cell types.

Type I pneumocytes form the alveolar wall and are essential for gas exchange, covering approximately 95% of the alveolar surface. Type I pneumocytes cannot replicate.

Type II pneumocytes secrete pulmonary surfactant, a lipoprotein that lowers surface tension, preventing alveolar collapse during exhalation. Additionally, type II pneumocytes function as progenitor cells, capable of differentiating into either type I or new type II pneumocytes in response to injury, which is crucial for maintaining the structural integrity and function of the alveoli.

Alveolar macrophages function as vital immune cells, clearing the alveoli of debris and pathogens through phagocytosis while producing cytokines and chemokines to recruit and activate other immune cells. These roles make them pivotal in maintaining both immune defense and tissue homeostasis. Alveolar macrophages are also essential for tissue remodeling and repair.

All cells from the mononuclear phagocyte system originate from hematopoietic stem cells in the bone marrow, which give rise to both myeloid and lymphoid progenitors. Common myeloid progenitors differentiate into various myeloid cells, including granulocyte-monocyte progenitors, which further develop into monocytes.

Monocytes circulate in the bloodstream and subsequently migrate into tissues, differentiating into macrophages. However, specific tissue-resident macrophages, including alveolar macrophages in the lungs, may develop on-site during embryonic stages and maintain their population independently of circulating monocytes through local proliferation. Monocytes have a short lifespan in the blood, typically around 1 to 3 days. In contrast, tissue macrophages, such as alveolar macrophages, can persist for months to years under steady-state conditions.