Explain the process of phagocytosis

Phagocytosis is a non-specific cellular defence mechanism by the body which is initiated when a pathogen invades the body and has antigens which are detected as antigenic or foreign – note that antigens are simply just glycoproteins or proteins which protrude out of the plasma membrane which the body uses as a recognition molecule to detect whether something is ‘self’ or ‘non self’. It is the antigen that triggers phagocytosis and other immune responses, however, the antigen alone does not cause disease. Once the pathogen invades the body, it releases chemical signals which are detected by macrophage cells, a type of phagocytic cell, which will use their chemoreceptors on their plasma membrane to locate the pathogen, ready to then engulf it. This is done by a process known as chemotaxis – taxis simply just meaning having a directional response to stimuli, in this case, chemical signals. Once the pathogen is located by the macrophage cell, invagination of the membrane begins. The macrophage cell will engulf the pathogen as the membrane invaginates, forming a pouch /cavity around pathogen in a vesicle through a process known as endocytosis. The formation of the vesicle around the pathogen as the membrane of the macrophage cell engulfs it is referred to as a phagosome. The membrane changing shape is aided by the depolymerisation of actin, a key component to the cells cytoskeleton. Once inside the macrophage cell, hydrolytic enzymes from the cells lysosomes are added to the phagosome, forming a phagolysosome. The enzymes will then begin to hydrolyse (break down) the pathogen to its monomers e.g. amino acids and monosaccharides. Useful products like such will be used by the macrophage cell and stored for later use, yet harmless waste products from the pathogen are excreted in a vesicle via a process called exocytosis. However, the pathogens antigen instead of being released by the macrophage cell is transferred to macrophage cells plasma membrane and is displayed on a protein complex called the major histocompatibility complex (MHC). Here, the macrophage cell presents the antigen of the pathogen on its membrane, thus becoming a APC (antigen presenting cell) which will then go on to initiate an adaptive immune response, such as the cell mediated response or the humoral response.