Macrophages

Macrophages are a type of white blood cell (WBC) that play an important role in the immune system, particularly in a process called phagocytosis, where they engulf and digest pathogens (viruses and bacteria), dead cells, and cellular debris. They start their life as monocytes, a type of blood cell, that migrate from the blood to various tissues throughout the body where they differentiate (change) into macrophages.

What do macrophages do?

Macrophages have several key functions in both innate and adaptive immunity, as well as in tissue homeostasis:

• Phagocytosis: They are known for their ability to engulf and digest pathogens, dead cells, and other particles, a process crucial for clearing infections and damaged cells.
• Antigen presentation: After digesting a pathogen, macrophages can present pieces of the pathogen (antigens) on their surface to T cells, initiating a specific immune response.
• Cytokine production: Macrophages release cytokines, which are signaling proteins that help regulate the immune response, including inflammation. They can recruit other immune cells to sites of infection or injury and can modulate the activity of those cells.
• Tissue repair: Beyond their immune functions, macrophages play a role in promoting tissue repair and regeneration typically in response to inflammation. They help to clear dead cells and debris from injury sites and can secrete factors that promote healing.

Where are macrophages normally found?

Macrophages are found throughout the body reflecting their role in surveilling for and responding to injury and infection. They are particularly abundant in connective tissues, the spleen and lymph nodes (part of the lymphatic system), the lungs (alveolar macrophages), the liver (Kupffer cells), the brain and spinal cord (microglia), the skin, the lining of the digestive tract.

What do macrophages look like under the microscope?

Under the microscope, macrophages are large cells with a considerable amount of cytoplasm, making them larger than most other immune cells. Their nucleus is often indented or kidney-shaped and the cytoplasm may appear foamy or contain vacuoles due to the ingestion of foreign particles and debris.

Macrophages can adopt different activation states, reflected in their appearance and function, based on the signals they receive from their environment. For example, “classically activated” macrophages, induced by certain types of immune signals, are more aggressive in attacking pathogens, while “alternatively activated” macrophages, induced by different signals, are more involved in tissue repair and resolving inflammation. These different states can sometimes be distinguished by changes in their appearance under the microscope or by the presence of specific markers detectable with special stains or immunohistochemistry (IHC).

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