Phagocytes are white blood cells that use phagocytosis to engulf debris, particles and bacteria to protect the body. There are several types of phagocytes, all of which do a slightly different job within the body.
This article shall consider the different phagocytic cells present within the body, their structure, where they are located and clinical conditions that may result from a lack of them.
Monocytes are the largest type of phagocyte, with a kidney bean shaped nucleus when seen under a microscope, as well as a granular cytoplasm. They are produced in the bone marrow from their precursor, monoblasts. Monocytes are found in the blood stream, differentiating to macrophages and dendritic cells once they enter other tissues after one to three days. Their main role within the blood is phagocytosis, as well as antigen presentation to T cell and cytokine production.
Macrophages are derived from monocytes, once they move out of the blood and into other parts of the body. Their main role within many tissues is phagocytosis of cellular debris, microbes and any other foreign substances, most importantly in chronic inflammation. They also help initiate immune responses by presenting antigens to T cells. There are multiple different types of macrophage for different tissues, microglia in the CNS and Kupffer cells in the liver for example.
Dendritic cells are produced within bone marrow, and are released into the blood stream as immature dendritic cells. They can be recognised by the presence of multiple projections from their surface. This gives them a large surface area to volume ratio, allowing the cell to cover a large area in order to do carry out it’s function. That function is to ingest antigens by phagocytosis, then migrate to lymph nodes where they present antigens to T cells, activating the T cells to cause an immune response if necessary. They are most abundant in tissues that bridge external and internal environments, such as skin, GI tract and lungs.
Neutrophils are the most abundant type of white blood cell, and can be identified by their granular cytoplasm and lobulated nuclei (usually 2-5 lobules). They are formed from myeloid stem cells found within bone marrow. They are normally found within the bloodstream, but during the acute phase of infection they are among the first inflammatory cells to reach the site of infection. As well as phagocytosis, they can release molecules that can break down microbes. They also release cytokines to increase the inflammatory reaction and recruit more inflammatory cells.
Neutropenia, or low neutrophil count, can leave patients highly susceptible to infections, which shows the importance of neutrophils to tackling infection. Neutropenia often presents with symptoms of infection, such as fever, painful swallowing, skin abscesses, otitis and gingival pain. Children may also be irritable and show poor feeding.
Causes of chronic neutropenia include:
- Aplastic anaemia
- Glycogen storage disease
- Vitamin B12 deficiency
- Congenital immunological disorders
Cause of acute neutropenia include:
- Side effect of trimethoprim
It is diagnosed via a full blood count and said to be a neutrophil count of less than 1 x 109/L. If needed, a bone marrow biopsy can also be performed to aid diagnosis. Treatment can involve medication that aims to stimulate production of neutrophils, particularly in congenital causes. In addition, prophylactic antibiotics may be required. There is also the possibility of intravenous immunoglobulins having some success when treating this disorder.