Histiocytes

July 27, 2023

Histiocytes are a type of immune cell that are part of the body’s defense mechanism against infections and foreign substances. They belong to the mononuclear phagocyte system including monocytes in the blood and macrophages in tissues. Histiocytes play an important role in the immune system’s response to pathogens and tissue repair and maintenance. They are commonly seen in areas of chronic inflammation.

What do histiocytes do?

Histiocytes have several important functions in the body:

• Phagocytosis: They can engulf and digest cellular debris, foreign substances, microbes, and cancer cells, a process known as phagocytosis.
• Antigen presentation: Histiocytes process foreign materials and present them on their surface to T cells, facilitating the adaptive immune response.
• Secretion of cytokines: They produce cytokines, which are signaling molecules that help to modulate the immune response, attracting other immune cells to sites of infection or inflammation.

Where are histiocytes normally found?

Histiocytes are primarily found in the connective tissue of non-immune organs such as the skin, lungs, and digestive tract and in small immune organs called lymph nodes. As part of the mononuclear phagocyte system, they are distributed widely to surveil for and respond to infections or injury.

What do histiocytes look like under the microscope?

Under the microscope, histiocytes appear as large cells with a distinct, oval, or indented nucleus and abundant cytoplasm. The cytoplasm may contain vacuoles or granules due to their phagocytic activity. Histiocytes can differentiate into various cell types, including dendritic cells and macrophages, depending on the tissue context and signals they receive. In tissue sections stained with hematoxylin and eosin (H&E), histiocytes are identifiable by their size, the shape of their nucleus, and their cytoplasmic characteristics.

Their appearance can vary depending on their state of activation and the specific tissue they are located in. When activated, for example, during an infection or inflammation, they can increase in size and the number of granules or vacuoles in their cytoplasm may become more prominent, reflecting their enhanced phagocytic or antigen-presenting activities.

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