and neutrophils, as well as "activated" platelets, neutrophils and T-cells, are thought to be phagocytosed alive by macrophages. Development. Phagoptosis removes excess cells during development in the worm, C. elegans. During mammalian development multiple cells undergo programmed cell senescence and are then phagocytosed by macrophages. Brain macrophages (microglia) can regulate the number of neural precursor cells in the developing brain by phagocytosing these otherwise viable precursors and thus limiting neurogenesis. Turnover of blood cells. Red blood cells (erythrocytes) live for roughly 3 months in the blood before being phagocytosed by macrophages. Old erythrocytes do not die, but rather display changes in the cell surface that enable macrophages to recognise them as old or damaged, including exposure of phosphatidylserine, desialylation of glycoproteins, loss or changed conformation of the "don't-eat-me" signal CD47, and exposure of novel antigens that bind endogenous antibodies. Neutrophils have a daily rhythm of entry and exit from the blood, driven by neutrophil “aging” in the circulation, causing decreased expression of CD62L and increased expression of CXCR4, which directs the “aged” neutrophils to the bone marrow, where they are phagocytosed by macrophages. However, it is still unclear how or why neutrophils turnover at such an enormous rate. Antigen recognition causes phosphatidylserine exposure on activated T-cells, which is recognized by Tim-4 on macrophages, inducing phagoptosis of the activated T-cells, and thus the contraction phase of the adaptive response. Host defence against pathogens. Phagocytosis of otherwise-viable pathogens, such as bacteria, can be mediated by neutrophils, monocytes, macrophages, microglia and dendritic cells, and is central to host defence against pathogens. Dendritic cells can phagocytose viable neutrophils, and present antigens derived from bacteria or cancer cell debris previously phagocytosed by the neutrophils. Thus phagoptosis can contribute to host defence in a variety of ways. Host defence against cancer. It has been known for some time that animals defend themselves against cancer by antibody-mediated or antibody-independent phagocytosis of viable tumour cells by macrophages. Recognition of viable cancer cells for phagocytosis may be based on the expression of novel antigens, senescence markers, phosphatidylserine or calreticulin. More recently it has become clear that most human cancer cells overexpress CD47 on their surface to prevent themselves being phagocytosed, and that if this ‘don’t-eat-me’ signalling is blocked then a variety of cancers can be cleared from the body. Thus it would appear that phagoptosis is an important defence against cancer, but that tumour cells can suppress this, and