Final answer:
Monocytes, a type of white blood cell from myeloid stem cells, transform into macrophages upon reaching infected tissues. Macrophages, either roaming or fixed, are primary defenders in the immune response, phagocytizing debris and pathogens, and releasing cytokines to orchestrate further immune action.
Step-by-step explanation:
Monocytes are a type of white blood cell that circulate in the blood and lymphatic systems. Originating from myeloid stem cells, they account for 2-8% of the total leukocyte count and are identifiable by their large size and unique nuclear shape. Upon migrating to infected tissues, they differentiate into macrophages, becoming a key component of the immune response.
Macrophages are versatile phagocytes that roam through connective tissues (free macrophages) or stay fixed in one location (fixed macrophages). Their main function is to engulf and digest cellular debris, foreign pathogens, and other particles. This process is facilitated by their amoeboid movement and their ability to squeeze through capillary walls. In addition to their phagocytic role, macrophages release chemotactic chemicals and cytokines, which activate other immune cells and attract them to the site of infection or inflammation.
Particularly, cytokines secreted by Th1 cells can influence the accumulation and activation of macrophages at inflammatory sites, enhancing the immune response. Examples of tissue-specific macrophages include Kupffer cells in the liver, histiocytes in connective tissues, and alveolar macrophages in the lungs.