Yellow mucus results from a draining infection, typically in the sinuses or throat, though it can occur from infections elsewhere such as the head and ears. Mucus discharge is rarely serious but you should monitor the consistency and frequency to be safe. If yellow mucus drains from any part of your body after an injury, particularly to the head, it is important to see your doctor immediately because this could be a sign of something more serious.
The common cold is the most frequent cause of yellow or greenish coloured mucus discharge. Mucus is typically clear during the first days of a cold as your body is trying to "wash away" the bacteria by releasing discharge. When your white blood cells begin fighting back, your mucus will change from a clear to white or yellowish colour. Drink plenty of fluids to ease the passing of the excess mucus. There is no need for antibiotics but if your symptoms persist longer than two weeks, see your doctor to ensure the cold has not become a sinusitis or bronchitis.
Sinusitis, the infection of your sinuses, creates swelling and tenderness in your nasal passages. Your nose naturally produces mucus as a lubricant throughout the day but when drainage passages are blocked by swollen sinuses the mucus becomes thick. As the blockage continues and drips to the Eustachian tube, you may begin coughing yellow mucus from your throat. Anything from pollution to pollen in the air can cause sinusitis and although not all yellow mucus warrants antibiotics, see your doctor as he may prescribe them in the case of infection.
Allergic reactions to environmental triggers such as mould, pollen or animal dander cause a release of histamines in your body. Histamines signal a variety of reactive symptoms such as coughing, wheezing and mucus secretion. Nasal congestion will typically occur a few hours after exposure releasing a yellow mucus discharge. To avoid nasal congestion and other allergy symptoms talk with you doctor about taking an antihistamine or a prescription allergy medication, if necessary.