One of the basic immunological mechanisms in metazoans is the use of host cells for the removal of non-self, such as microbes. This type of immunological mechanism is known as cell-mediated immunity (CMI). All metazoan animals examined to-date appear to possess CMI. It appears to be an ancient mechanism and appears to have appeared early in evolution. Some single-celled organisms, such as the amoeba and the slime mold (Dictyostelium discoideum) possess the ability to 'deform' themselves so that they can ingest large particulate matter. This process, known as phagocytosis ("cell eating"), was the mechanism used by these cell types for obtaining nutrients from their external environments. With the evolution of complex multicellular body forms, animals developed organ systems that were dedicated for acquiring and processing nutrients. Although this made cell-based nutrition redundant, phagocytosis did not disappear from the animal lineages. Instead, phagocytic cells (also known as phagocytes) became co-opted into immunological roles such as immunosurveillance and elimination of non-self. Indeed, specialised phagocytes are integral components of sophisticated immune systems and have co-evolved with other immunological components to mediate elaborate responses against infections.


Sham Nair 2014