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Explain what phagocytosis is including the role of neutrophils and macrophages.

Explain what phagocytosis is including the role of neutrophils and macrophages
Phagocytosis is a process used by cells to engulf and subsequently ingest particles of nutrients or bacteria. This process is a very important part of cell function, allowing cells to grab vital nutrients and allowing the body to protect itself from harmful bacteria. A cell that specializes in this process is known as a phagocyte. It is one among a family of processes collectively referred to under the blanket term “endocytosis,” which refers to any sort of ingestion of material by a cell. The opposite is exocytosis, the expulsion of unwanted material from a cell.In this process, a cell deforms its membrane to form a little cone around the piece of material that is to be absorbed, and then it closes the sides of the cone, hugging the particle in the cell membrane to create what is known as a phagosome or food vacuole, like a little envelope of material surrounded by the cell membrane. The phagosome, in turn, is passed into the cell for absorption by the lysosomes, the cell structures that specialize in digesting materials that enter the cell. The lysosomes break the phagosome down into its component materials, passing useful compounds on to other structures in the cell and expelling the rest as waste material. In the case of some infectious or harmful material, the phagosome may enter a peroxisome, a special cell structure that helps to rid the body of toxins.In unicellular organisms, phagocytosis is a critical function, as, without it, the organism will not survive. Some of these organisms have adapted special traits that allow them to track food, orienting themselves in the direction of useful particles that they can ingest. In multicellular organisms, it tends to be a more passive process, but it is still crucial for the survival of the individual cells, ensuring that they get the nutrients they need to function.

Immune system cells also perform phagocytosis, trapping harmful materials when they enter the body and destroying them so that they cannot cause damage. In some cases, the body may respond with inflammation, as a flood of immune system cells rushes to a location to deal with unwanted invaders. When the immune system can no longer cope with harmful materials, or when these materials resist the ingestion process, the consequences for the host body can be quite unpleasant.
the role of neutrophils and macrophages
1. Neutrophils are primarily circulating phagocytes recruited from the blood to sites of inflammation during acute events where they phagocytose opsonised organisms and dead cells. Macrophages are derived from blood monocytes and are found resting in the tissues. These would include Kuppfer cells in the liver and alveolare macrophages in the lung. Macrophages also are excellent at phagocytosis but upon activation (usually occuring via interferon gamma priming from NK T cells combined with a secondary stimulus such as bacterial Lipopolysaccharide acting through CD14 or Toll like receptor 4) they secrete pro-inflammatory cytokines such as IL-1, IL-6, IL-12 and IL-18 and TNF alpha and other molecules such as procoagulant tissue factor. Under certain circumstances they can also secrete anti-inflammatory cytokines such as IL-10. It is this role as a cytokine factory that makes macrophages centrally important in the regulation of both the innate and adaptive immune system. The wide variety of secretory products makes them stand out from the relatively 'simple' neutrophil.

2. The adaptive immune response resulting in the production of antibody is mediated by antigen presentation by cells such as dendritic cells and indeed B cells (on MHC II) to CD4 bearing T helper cells. These Helper T cells when activated secrete cytokines and express co-stimulatory molecules allowing them to interact with antigen specific B cells and stimulate the production of antigen specific antibody which can then act as an opsonin. Phagocytes such as neutrophils arent really involved although macrophages can act as antigen presenting cells.

. Antigen presentation does not necessarily have to involve phagocytosis. Dendritic cells constantly sample the extracellular environment by pinocytosis and present processed antigen on MHC molecules. Also there are other molecules such as complement C3b which can act as opsonins (or labels for phagocytosis) even in the absence of antibody.

Try not to think of the immune system as a series of discrete processes or cells. Remember these things are all happening in an orchestrated regulated fashion simultaneously not individually.

see also for further understanding

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