Phagocytosis is the name of the process by which a white blood cell engulfs and destroys a microorganism such as a bacterium. The white blood cells that carry out phagocytosis are collectively known as phagocytes, and they include macrophages, neutrophils and dendritic cells.
When a phagocyte encounters a bacterium in the body, the immune cell first sends out projections to surround and capture the invading organism. The white blood cell creates a vesicle around the bacterium and pulls it into the cell. Small vesicles filled with lysozymes merge with the vesicle holding the captured bacterium and the lysozymes then break down and destroy the microorganism. Most of the leftover bacterial particles are consumed by the cell, but some phagocytes send bacterial antigens to the cell surface to serve as signals so that other immune cells can learn which bacterial invaders the phagocyte has encountered. While they are engulfing and consuming an invading microorganism, phagocytes also release signal molecules to help coordinate the overall immune response.
Not all white blood cells perform phagocytosis. Some spray bacteria-destroying chemicals directly onto invading organisms instead of engulfing them first. Others release compounds that cause inflammation, which draws other immune cells to the area. The B-cells initially perform phagocytosis to identify pathogens, but once the invader is identified, B-cells shift to building and releasing antibodies instead.