A phagocyte is a type of white blood cell that helps the human body fight off infection and disposes of dead or dying somatic cells. Phagocytes rid the body of bacteria and other pathogens via an ingestion process called phagocytosis. During phagocytosis, phagocytes engulf and kill microbes using a variety of different methods. There are two types of phagocyte, professional and non-professional. The professional cells are equipped with receptor molecules that are attracted to certain chemicals that signal the presence of an infection.
One of the important roles filled by phagocytes is the disposal of cells that have undergone apoptosis, or programmed cell death. Dead or dying cells are disposed of by nonprofessional phagocytes. The cells put out chemical signals that allow the phagocyte to detect their decline, so it can then ingest the dying cells using phagocytosis. Professional phagocytes also use phagocytosis to dispose of bacteria and other microbes. Viruses cannot be killed using this process, as they use phagocytosis to invade white blood cells and infect the rest of the body.
Phagocytosis begins with the phagocyte surrounding the microbe or dead cell. When the harmful cell is completely engulfed in the phagocyte, it is trapped inside a chamber called a phagosome or phagocytic vesicle. Enzyme-containing organelles called lysosomes then fuse with the phagosome, creating a structure called a phagolysosome, in which the trapped particle is killed and digested.
Phagocytes can kill microbes using either intracellular or extracellular processes. The most efficient killing process takes place within the phagocyte and depends on molecules that contain oxygen found within the white blood cell. Oxygen radicals undergo various chemical reactions in the presence of enzymes found in the phagolysosome. These chemical reactions convert the oxygen to hydrogen peroxide and singlet oxygen, a less stable form of oxygen molecule. Hydrogen peroxide is an antiseptic and disinfectant that kills microbes.
There are also types of intracellular killing that do not rely on the presence of oxygen. Antimicrobial proteins in the phagolysosome can also kill bacteria by attacking their bacterial membranes. Binding proteins called lactoferrins deprive bacteria of iron that the microbes need to grow and reproduce. Extracellular killing occurs outside the cell, and depends on the presence of a protein called Interferon-gamma. This protein activates a professional phagocyte called a macrophage so it produces another protein called tumor necrosis factor that causes cell death.
Professional phagocytes come in many types. There are neutrophils, which are the most numerous type of phagocyte and are usually the body's first line of defense against infection. Macrophages are usually stationary, or "fixed," when they reach maturity, guarding vital areas of the body like the liver, lungs, and brain. Dendritic cells receive their name from growths called dendrites that they produce. Monocytes, aside from performing phagocytosis, also replenish macrophages and dendritic cells in a healthy body.
