Phlegm and mucus are terms often used interchangeably, but they refer to different substances in the body. Mucus is a slippery, gel-like substance produced by the mucous membranes lining various body cavities, such as the respiratory tract, digestive system, and reproductive organs. It serves as a protective barrier and helps trap foreign particles, preventing them from reaching and damaging sensitive tissues. Phlegm, on the other hand, is a specific type of mucus produced in the respiratory tract, particularly in the lungs and the throat. It is thicker and often appears as a viscous, sticky material when coughed up or expelled during respiratory infections. Phlegm may contain inflammatory cells, bacteria, or viruses, and its production increases as a response to respiratory infections or irritants.