According to the knowledge of biology, how is phlegm formed?

As we all know, the inner walls of trachea and bronchus are covered with a layer of mucosa, which is composed of ciliated columnar epithelium and goblet cells. There are many mucous glands and serous glands in the submucosa, and the glandular ducts open on the mucosal surface. Under normal circumstances, goblet cells and glands secrete a small amount of mucus to cover the mucosal surface, which can protect the mucosa, keep the tracheal mucosa moist, make dust particles and bacteria inhaled into trachea and bronchus adhere, and prevent them from entering the depths of lung tissue. Then, with the help of cilia of ciliated columnar epithelium, it is discharged to the larynx at the upper end of trachea and coughed up through the mouth, which is sputum. When the trachea, bronchi and lungs are stimulated by harmful factors or infected by pathogenic bacteria, the respiratory mucosa is congested and edematous, a large number of inflammatory cells infiltrate, blood vessels dilate and exudation increase, goblet cells and submucosal glands in the mucosa proliferate and hypertrophy, and mucus secretion increases greatly, which is beneficial to the removal of foreign bodies. Excessive mucus secretion increases the burden of ciliated columnar epithelium, which is not conducive to mucus discharge. Under the action of bacteria and their toxins, some denatured and necrotic tissue cells are produced and remain in the bronchus, and mucus and these denatured and necrotic tissue cells constitute sputum. Therefore, when treating cough, it is necessary to cooperate with some expectorants, such as fresh bamboo juice and expectorant spirit, to help the human body expel phlegm.