Phagocytosis

Phagocytes are white blood cells that engulf and digest pathogens. The two main types are neutrophils, which have a lobed nucleus and are the first to arrive at an infection, and macrophages, larger cells that develop from monocytes and live in tissues.Phagocytosis happens in stages:
1. Attraction: chemicals released by pathogens and damaged cells attract phagocytes to the site of infection.
2. Recognition and attachment: receptors on the phagocyte bind to non-self antigens on the pathogen. Binding is easier if the pathogen is coated with antibodies or complement proteins, which act as opsonins.
3. Engulfing: the phagocyte's cell surface membrane extends around the pathogen and encloses it in a vesicle called a phagosome (phagocytic vacuole). This is a type of endocytosis.
4. Digestion: lysosomes move to and fuse with the phagosome. They release lysozymes and other hydrolytic enzymes that break down the pathogen.
5. Removal: soluble products are absorbed, and any waste is released by exocytosis.Neutrophils usually die after engulfing pathogens; dead neutrophils, bacteria and tissue fluid make up pus.Macrophages do not fully destroy the antigens. They place fragments of the pathogen's antigens on MHC proteins on their cell surface membrane and become antigen-presenting cells (APCs). This links the non-specific response to the specific response, because T helper cells recognise the presented antigens.