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Yellow phlegm in particular can scream infection. But, interestingly enough, the color does not come from infection. A yellow hue to the mucus is caused from white blood cells, according to Health.com. So, yellow mucus therefore isnât a result of an infection, but a result of the body fighting an infection. Coughing up yellow mucus therefore can be associated with battling the illness as opposed to being a symptom.
This means that the inflammation of the air passages is due to an infection. Coughing up yellow to yellowish brown phlegm is common in most acute respiratory conditions. It is actually the presence of certain immune cells, the white blood cells such as neutrophils and eosinophils, that phlegm may take on a yellow color.nfections of the respiratory passages is the most common cause of coughing up of phlegm. Most cases are acute and resolve without any complications. 1  Acute bronchitis is usually bacterial in nature and tends to develop after the common cold or flu. 2  It is more frequently seen in cigarette smokers.