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Phlegm (/ˈflɛm/; Greek: φλέγμα "inflammation, humour caused by heat") is a liquid secreted by the mucous membranes of mammalians. Its definition is limited to the mucus produced by the respiratory system, excluding that from the nasal passages, and particularly that which is expelled by coughing (sputum). Phlegm is in essence a water-based gel consisting of glycoproteins, immunoglobulins, lipids and other substances. Its composition varies depending on climate, genetics, and state of the immune system. Its colour can vary from transparent to pale or dark yellow and green, from light to dark brown, and even to dark grey depending on the constituents.

Contrary to popular misconception and misuse, mucus and phlegm are not the same