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Both monocytes and macrophages are types of white blood cells that are part of the immune system. Monocytes are found mostly within the blood, while macrophages occur within the tissues. Macrophages are created through the differentiation of monocytes. In most cases, monocytes float within the blood stream for up to three days before migrating to a specific tissue for macrophage differentiation.
Monocytes are created within the bone marrow and are free floating within the blood stream. They have different roles depending on whether the body is undergoing an immune response or not. When there is no immune response, monocytes move to different tissues where they develop into macrophages and dendritic cells. In this case, the monocytes are replacing any macrophages that have died off.
If there is an immune response by the body, monocytes can rapidly move to the site of the infection and take an active and passive role in the response. Actively, monocytes can attack the foreign bodies within the blood stream and the tissues. Otherwise, they differentiate into macrophages within the tissue so that there are more of these types of cells to carry out the immune response.
During an immune response by the body, monocytes and macrophages carry out phagocytosis. In this process, large particles are taken up by the cells. There are two reasons for phagocytosis, which are to protect the body against harmful foreign molecules — or pathogens, and to get rid of damaged, dying and dead cells. In addition to carrying out phagocytosis to rid the body of pathogens, monocytes and macrophages have two other roles as part of the immune system: antigen presentation and cytokine production.
Antigen presentation occurs when parts of the pathogen that has been engulfed are displayed on the surface of the monocytes and macrophages. The importance of antigen presentation is that it activates T cells so they can recognize other molecules as foreign bodies in the future. This allows the immune system to react quicker when it comes in contact with foreign molecules that also have the antigen protein structure on their membranes.
Cytokines are proteins that are used to signal other cells during cellular communication. They act in similar ways to hormones and can stimulate and inhibit certain processes within cells. Common cytokines produced by monocytes and macrophages include tumor necrosis factor (TNF) and interleukins. TNF causes cell death, while interleukins are involved in the symptoms associated with an inflammation, including fever.