Macrophages are specialized leukocytes that respond to invading pathogens by initiating phagocytosis and the synthesis and release of pro-inflammatory cytokines. Microorganisms like bacteria have pathogen-associated molecular patterns (PAMPs) that are identified by pattern recognition receptors on macrophages. The left side of the figure illustrates the process of phagocytosis, which involves engulfment of the bacterium into an intracellular vesicle called a phagosome, phagosome-lysosome fusion to form a phagolysosome, degradation of the bacterium by enzymes, and cellular release of the degraded material by exocytosis. The right side of the figure illustrates that bacterial binding to surface receptors of the macrophage also signals the transcription of pro-inflammatory cytokines in the cellís nucleus. Cytokines are then produced in the cytoplasm and these pro-inflammatory proteins are secreted from the cell to affect behavior of nearby cells.