Phlegm is a substance similar to mucus that occurs in the lower respiratory system leading to the lungs, states Everyday Health. Unlike mucus, phlegm doesn't typically appear in the throat and sinuses unless a person coughs it up. Coughing up phlegm is usually a symptom of pneumonia or bronchitis.
Mucus is a sticky, gelatinous substance that acts as a filter for the lungs, according to Everyday Health. Produced in the sinuses, mucus catches dirt and debris inhaled through the sinus passages. It may be thin and watery when a person suffers from a cold, causing the nose to run, or it may be thick and dry, which leads to congestion and post-nasal drip. Yellowish-green mucus or phlegm may be a symptom of viral infection. A person should seek medical attention if coughing up phlegm with blood in it.
Mucus also lubricates membranes in the mouth and nose, keeping them from drying out, reports Discovery News. Made from water, polysaccharides and proteins, it provides a protective coating for the sinus and mouth membranes. When a person gets sick, mucus production increases in order to assist with flushing out bacteria and virus contaminants. A person can manage excess mucus by staying hydrated, taking a hot shower or a steam bath, gargling with salt water, or taking drugs such as Mucinex.