Phagocytes and Their Relatives
Phagocytes are large white cells that can swallow and digest microbes and other foreign particles. Monocytes are phagocytes that circulate in the blood. When monocytes migrate into tissues, they develop into macrophages. Specialized types of macrophages can be found in many organs, including the lungs, kidneys, brain, and liver.
Macrophages play many roles. As scavengers, they rid the body of worn-out cells and other debris. They display bits of foreign antigen in a way that draws the attention of matching lymphocytes and, in that respect, resemble dendritic cells. And they churn out an amazing variety of powerful chemical signals, known as monokines, which are vital to the immune response.
Granulocytes are another kind of immune cell. They contain granules filled with potent chemicals, which allow the granulocytes to destroy microorganisms. Some of these chemicals, such as histamine, also contribute to inflammation and allergy.
One type of granulocyte, the neutrophil, is also a phagocyte. Neutrophils use their prepackaged chemicals to break down the microbes they ingest. Eosinophils and basophils are granulocytes that “degranulate” by spraying their chemicals onto harmful cells or microbes nearby.
Mast cells function much like basophils, except they are not blood cells. Rather, they are found in the lungs, skin, tongue, and linings of the nose and intestinal tract, where they contribute to the symptoms of allergy.
Related structures, called blood platelets, are cell fragments. Platelets also contain granules. In addition to promoting blood clotting and wound repair, platelets activate some immune defenses.
Dendritic cells are found in the parts of lymphoid organs where T cells also exist. Like macrophages, dendritic cells in lymphoid tissues display antigens to T cells and help stimulate T cells during an immune response. They are called dendritic cells because they have branchlike extensions that can interlace to form a network.
Phagocytes, granulocytes, and mast cells, all with different methods of attack, demonstrate the immune system’s versatility. View credit information.
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