Phagocytosis is a process used by certain cells to absorb and ingest solid particles. The particles that are absorbed and ingested can be nutrient particles or bacteria. The cells capable of ingesting by the phagocytosis process are known as phagocytes. Phagocytes can be free-living unicellular organisms, such as amoeba, or they can be a part of the body cells of a multicellular organism. Phagocytes belong to the family of processes that are collectively referred to as 'endocytosis', consisting of all the processes that involve ingestion of material by cell. The phagocytosis process is a specific type of endocytosis that involves vesicular internalization of solid particles, such as bacteria, unlike other endocytic processes that involve vesicular internalization of liquids. Certain unicellular organisms, such as the protists, use this particular process as a means of feeding. It provides them a part or all of their nourishment. This mode of nutrition is known as phagotrophic nutrition. In amoeba, this process takes place by engulfing the nutrient with the help of pseudopodia that are present all over the cell, whereas in ciliates, a specialized groove or chamber known as the cytostome is present where the process takes place. Other examples of phagocytosis include some immune system cells that engulf and kill certain harmful, infectious microorganisms, and other unwanted foreign materials. Mammalian immune system contains certain phagocytes that helps them destroy and get rid of pathogenic bacteria and other infectious organisms. In these cells, the engulfment of foreign material is facilitated by actin-myosin contractile system. It allows the cell membrane to expand in order to soak in the particle and then contract immediately, ingesting it. It is quite fascinating to watch this process under a microscope. One can actually see the phagocyte absorbing the foreign particles. There are several distinct steps involved in the process of phagocytosis. Let us have a look at each of them. In case of unicellular organisms, the process of phagocytosis takes place when the organism comes in contact with nutrient particles. The phagocytes in the immune system are activated in the presence of certain bacterial cells, inflammatory cells, or other foreign bodies. Let us talk about the general steps involved in the process. Step 1: The phagocyte gets actuated by the presence of certain particles around it. As soon as it detects a foreign particle, the phagocyte produces surface glycoprotein receptors that increase its ability to adhere to the surface of the particle. Step 2: The phagocyte slowly attaches to the surface of the foreign particle. The cell membrane of the phagocyte begins to expand and forms a cone around the foreign particle. Step 3: The cell membrane surrounds the foreign particle from all sides to create a vacuole, known as phagosome or food vacuole. The phagosome is then passed into the cell for absorption. Step 4: Now occurs the role of the lysosomes, which are cell structures specialized in digesting the particles that enter the cell through the cell membrane. The lysosomes break the food vacuole or phagosome into its component materials. The essential nutrients, if any, are absorbed in the cell, and the rest is expelled as waste matter. In case of the immune system, the cell creates a peroxisome, a special structure that helps the body to get rid of the toxins. In unicellular organisms like amoeba, the process of phagocytosis is necessary for survival, as they are totally dependent on it for nutrients. Some of these organisms have adapted special traits which enable them to locate and keep a track of food particles.