Asthma is a chronic respiratory condition characterized by inflammation and narrowing of the airways. A common and often challenging symptom for individuals with asthma is increased mucus production. This excess mucus can significantly contribute to breathing difficulties.
Mucus in Healthy Airways and Asthma
In healthy airways, mucus serves as a protective barrier, forming a thin layer that traps inhaled particles, allergens, and infectious agents. This mucus layer, composed of glycoproteins (mucins), proteins, and lipids, is continuously moved towards the throat by tiny hair-like structures called cilia, a process known as mucociliary clearance. This mechanism helps keep the lungs clean.
In contrast, mucus in asthmatic airways becomes more abundant, thicker, and stickier. This altered mucus is less effectively cleared, leading to its accumulation within the airways. This shifts mucus from a beneficial protective function to a problematic symptom in asthma.
The Inflammatory Process
Chronic inflammation is a primary driver of increased mucus production in asthma. This inflammatory response involves various immune cells, particularly T-helper 2 (Th2) lymphocytes and eosinophils, which infiltrate the airway walls. These cells release inflammatory mediators, such as cytokines, which are signaling proteins that coordinate immune responses.
Key cytokines implicated in mucus overproduction include interleukin-13 (IL-13) and interleukin-9 (IL-9). IL-13, a central mediator, directly stimulates airway epithelial cells, specifically goblet cells, to produce more mucus. This stimulation leads to goblet cell hyperplasia (an increase in mucus-producing cells) and hypertrophy (an increase in their size). Submucosal glands can also become hypertrophied and contribute to excess mucus secretion.
The increased activity of goblet cells and submucosal glands results in the overexpression of mucin genes, such as MUC5AC and MUC5B, which encode the main gel-forming proteins of mucus. While MUC5B is a principal mucin in healthy airways, MUC5AC production is significantly upregulated in asthma, contributing to the altered properties of asthmatic mucus.
External Factors That Trigger Mucus Production
Various external and internal factors can initiate or worsen the inflammatory process, thereby increasing mucus production in asthma. Common allergens such as pollen, dust mites, and pet dander are frequent triggers.
Environmental irritants also play a role, including smoke, air pollution, and strong odors from perfumes or household cleaners. These irritants can directly stimulate the mucous membranes in the airways, causing them to secrete more mucus than usual. Respiratory infections, particularly viral infections like the common cold or flu, are significant triggers, especially in children, as they can exacerbate inflammation and mucus production. Other factors, such as exercise and exposure to cold or dry air, can also induce an inflammatory response and contribute to increased mucus.
Consequences of Excessive Mucus
Excessive mucus buildup in asthmatic airways causes airway obstruction. This thick, sticky mucus can form plugs that block the bronchial tubes, making it difficult for air to move in and out of the lungs. Such obstruction contributes to common asthma symptoms including coughing, wheezing, and chest tightness.
The presence of excessive mucus can also create an environment conducive to bacterial infections. Mucus that is not efficiently cleared can trap pathogens, allowing them to multiply and potentially lead to secondary infections. The increased stickiness of asthmatic mucus, partly due to altered mucin composition, further impairs its clearance and exacerbates these issues.