The lining of your airways and lungs is built from a surprisingly diverse cast of specialized cells, each performing a distinct job that keeps you breathing safely. From the nose down to the deepest air sacs, at least a dozen recognized cell types work together to trap debris, fight infection, regulate fluid balance, and exchange oxygen for carbon dioxide. The tissue changes character as it descends through the respiratory tract, and some of its most important inhabitants were only identified in the last few years.
How the Lining Changes from Nose to Alveoli
The respiratory epithelium is not one uniform sheet. Most of the airway, from the nasal cavity through the trachea and into the bronchi, is covered by a tall, column-shaped tissue where every cell touches the basement membrane but not every cell reaches the surface. This gives it a layered appearance under the microscope even though it is technically a single layer. Farther down, in the smaller bronchioles, the lining transitions to shorter cells. At the very end, in the alveoli where gas exchange happens, the tissue thins dramatically into flat, delicate cells that allow oxygen and carbon dioxide to pass through with minimal resistance.1PubMed. Histology, Respiratory Epithelium Understanding which cell types live in each of these zones explains why certain diseases hit specific parts of the lung.
Ciliated Cells and the Mucociliary Escalator
Ciliated cells are the workhorses of the conducting airways and the most abundant cell type in the upper respiratory epithelium. Each one sprouts hundreds of tiny hair-like projections called cilia on its surface. These cilia beat in coordinated, wave-like motions that sweep a thin blanket of mucus, along with any trapped bacteria, dust, or pollen, steadily upward toward the throat, where you unconsciously swallow or cough it out. The system depends on three components working together: the sticky mucus layer on top, a watery layer beneath it that lets the cilia move freely, and the cilia themselves.2Cold Spring Harbor Perspectives in Biology. Cilia and Mucociliary Clearance
When cilia malfunction, the consequences are severe. People born with primary ciliary dyskinesia have structurally abnormal cilia that cannot beat properly, leading to chronic infections in the lungs, sinuses, and ears from childhood onward.2Cold Spring Harbor Perspectives in Biology. Cilia and Mucociliary Clearance Cigarette smoke also paralyzes cilia temporarily, which is one reason smokers are more prone to respiratory infections. The mucociliary escalator is so important that it is considered one of the three pillars of airway barrier function, alongside the physical seals between cells and the antimicrobial molecules the epithelium secretes.3PubMed Central. Barrier function of airway tract epithelium
Goblet Cells and Mucus Production
Goblet cells are the mucus factories of the airway. Named for their wine-glass shape, they are scattered among the ciliated cells in the upper airways and produce the gel-like mucus that lines the surface. This mucus traps inhaled particles and pathogens before the cilia push them out. The two key gel-forming proteins in airway mucus are MUC5AC and MUC5B, and the balance between them matters a great deal. In healthy airways, MUC5B dominates and plays a critical housekeeping role. In asthma, the expression of both MUC5AC and MUC5B shifts, contributing to the thick, sticky mucus plugs that narrow the airways during an attack.4Europe PMC. Airway Mucus and Asthma: The Role of MUC5AC and MUC5B.
Goblet cell numbers are not fixed. Chronic irritation from pollution, smoke, or allergic inflammation can cause the epithelium to produce more goblet cells at the expense of ciliated cells, a process called goblet cell hyperplasia. This tips the balance toward more mucus production and less mucus clearance, a combination that worsens conditions like chronic bronchitis.
Club Cells and Their Protective Secretions
Club cells, formerly known as Clara cells, are dome-shaped secretory cells concentrated in the bronchioles, the narrower airways where goblet cells become scarce. They produce a protein called club cell secretory protein (CC16), which has anti-inflammatory and protective effects. Research in mouse models has shown that lacking CC16 substantially worsens lung damage from cigarette smoke. Mice without the CC16 gene developed roughly twice as much airspace destruction and about 53% more small airway remodeling compared to normal mice after six months of smoke exposure.5European Respiratory Journal. A Protective Role For Club Cell Secretory Protein-16 (CC16) In The Development of Chronic Obstructive Pulmonary Disease (COPD)
Beyond CC16, club cells also help detoxify inhaled chemicals and can serve as progenitor cells in the bronchioles, dividing to replace damaged neighbors. In people with COPD, club cell numbers decline and CC16 levels in the blood drop, which may accelerate disease progression. Measuring CC16 in blood has been explored as a biomarker for lung health, though it has not entered routine clinical use.
Basal Cells as Airway Stem Cells
Basal cells sit along the basement membrane beneath the taller ciliated and secretory cells. They are small, inconspicuous, and easy to overlook, but they are arguably the most important cell type for keeping the airway intact over a lifetime. Lineage-tracing studies in mice have shown that basal cells function as genuine stem cells: they self-renew and generate the differentiated cell types of the airway, including ciliated cells, during both normal tissue turnover and repair after injury. Human airway basal cells behave the same way in laboratory assays, self-renewing and producing functional luminal daughter cells.6PubMed Central. Basal cells as stem cells of the mouse trachea and human airway epithelium
This stem-cell capacity makes basal cells central to how the airway recovers from infections, chemical injuries, or surgical wounds. Evidence from transplantation and long-term tracing experiments supports the idea that at least a subset of basal cells in the trachea and large bronchi maintains the tissue over the long term.7Disease Models & Mechanisms. Airway basal stem cells: a perspective on their roles in epithelial homeostasis and remodeling When basal cells themselves are damaged or depleted, as happens in severe burns or radiation injury, airway repair stalls, sometimes leading to scarring and permanent narrowing.
The Gas-Exchange Surface in the Alveoli
At the terminus of the airway tree sit roughly 480 million alveoli, thin-walled sacs where the actual business of breathing takes place. Two distinct epithelial cell types share this territory, and they could hardly be more different from each other.
Type I alveolar cells are extraordinarily flat and thin, stretched across more than 95% of the alveolar surface area despite making up a minority of alveolar cells by number.8PubMed Central. Pulmonary alveolar type I cell population consists of two distinct subtypes that differ in cell fate Their extreme thinness is what allows oxygen to diffuse from inhaled air into the blood, and carbon dioxide to move the other way, in a fraction of a second. For decades, type I cells were viewed as passive structural elements, but recent work has revealed that they retain some developmental flexibility and may participate in regeneration after injury.
Type II alveolar cells are cuboidal, smaller, and more numerous. Their defining job is manufacturing pulmonary surfactant, a mixture of lipids and proteins that coats the alveolar surface and dramatically reduces surface tension. Without surfactant, the alveoli would collapse on each exhale, which is exactly what happens in premature infants whose type II cells have not matured enough to produce adequate amounts.9PubMed. Regulation of surfactant secretion in alveolar type II cells Surfactant is assembled in specialized compartments within the cell and delivered to the surface in a tightly regulated process.10PubMed Central. Alveolar surfactant homeostasis and the pathogenesis of pulmonary disease
Type II cells double as the stem cells of the alveolar region. After lung injury, they proliferate and then flatten out to become new type I cells, restoring the gas-exchange surface.11PubMed. Repair and regeneration of the alveolar epithelium in lung injury This regenerative capacity is critical, but it has limits. Severe or repeated injury can exhaust the type II cell pool, contributing to conditions like pulmonary fibrosis.12PubMed Central. Function of epithelial stem cell in the repair of alveolar injury
A Transitional State Between Type II and Type I
The conversion from type II to type I is not instantaneous. Researchers have identified an intermediate cell state marked by high expression of the protein Krt8. These transitional cells have a flat shape reminiscent of type I cells but retain molecular signatures of stress, including activation of pathways linked to cellular aging. In animal models of lung injury, this Krt8-positive state appears consistently regardless of how the injury was caused, suggesting it is a fundamental step in alveolar repair. The same transitional cells have been found in human lung tissue from patients with pulmonary fibrosis, where they persist rather than completing their conversion, potentially contributing to scarring.13PubMed Central. Alveolar regeneration through a Krt8+ transitional stem cell state that persists in human lung fibrosis The discovery of this bottleneck has opened new lines of research into why some lungs heal cleanly while others develop fibrosis.
Pulmonary Neuroendocrine Cells
Scattered sparingly through the airway epithelium are pulmonary neuroendocrine cells, or PNECs, which blend features of nerve cells and hormone-producing cells. They act as sensors, detecting changes in the airway environment, such as drops in oxygen levels, rises in carbon dioxide, or mechanical stretch, and responding by releasing signaling molecules like serotonin and neuropeptides.14PubMed Central. Pulmonary neuroendocrine cells: crucial players in respiratory function and airway-nerve communication PNECs sometimes cluster into structures called neuroepithelial bodies, which sit at airway branch points and respond to multiple stimuli simultaneously, including low oxygen, high carbon dioxide, and physical distension of the airway wall.15PubMed. Pulmonary neuroepithelial bodies as airway sensors: putative role in the generation of dyspnea
When oxygen levels drop, specific ion channels on PNECs trigger serotonin release, which can influence local blood vessel tone and breathing reflexes.16PubMed Central. Pulmonary neuroendocrine cells: physiology, tissue homeostasis and disease PNECs are also of interest in cancer biology, because small cell lung cancer, one of the most aggressive forms of lung cancer, arises from neuroendocrine cells or their precursors.
Ionocytes and the Cystic Fibrosis Connection
Ionocytes were essentially unknown in the lung until 2018, when two research groups independently identified them using single-cell sequencing. Despite being extremely rare, ionocytes turned out to be the dominant source of CFTR transcripts in both mouse and human airways. CFTR is the gene mutated in cystic fibrosis, and its protein product is an ion channel that regulates chloride and fluid movement across the epithelium. When researchers knocked out the gene that drives ionocyte development in mice, CFTR expression collapsed and the animals developed hallmark features of cystic fibrosis, including dehydrated airway surfaces and abnormal mucus.17PubMed Central. A revised airway epithelial hierarchy includes CFTR-expressing ionocytes
The finding was startling because for decades, researchers had assumed CFTR’s most important home was in the ciliated cells and submucosal gland cells. The discovery raised the possibility that a tiny population of cells, making up perhaps one percent of the epithelium, could be the key driver of a disease that affects tens of thousands of people worldwide. However, the picture has grown more complicated. More recent work showed that ionocytes do regulate the acidity of the airway surface liquid in human bronchial tissue, but direct evidence that they are the main cells responsible for salt and water transport remains debated.18American Journal of Respiratory and Critical Care Medicine. Pulmonary Ionocytes Regulate Airway Surface Liquid pH in Primary Human Bronchial Epithelial Cells The field is still working out how much of the cystic fibrosis puzzle ionocytes actually explain versus how much belongs to CFTR activity in other cell types.
Tuft Cells and Airway Immunity
Tuft cells, named for the brush-like tuft of stiff microvilli on their surface, are another rare epithelial cell type only recently appreciated in the airways. They were better known in the gut, where they sense parasites and kick-start immune responses. In the lungs, tuft cells serve as chemosensory sentinels. When activated by allergens or other stimuli, they release a cocktail of immune-signaling molecules, including interleukin-25, cysteinyl leukotrienes, and acetylcholine.19PubMed. Chemosensory cells in the respiratory tract as crucial regulators of innate immune responses These signals activate innate immune cells and ramp up mucociliary clearance, effectively sounding an alarm and speeding up the system that expels the threat.
Tuft cells are the dominant source of interleukin-25 in the airway epithelium, and research in mouse models has demonstrated that their cysteinyl leukotrienes and IL-25 work together synergistically to trigger a powerful type 2 immune response, the branch of immunity involved in allergic inflammation.20PubMed Central. Tuft cell-produced cysteinyl leukotrienes and IL-25 synergistically initiate lung type 2 inflammation This makes tuft cells both protective, in that they help detect and expel threats, and potentially problematic in people prone to asthma or allergic airway disease, where type 2 inflammation spirals out of control.21PubMed. Tuft cells – the immunological interface and role in disease regulation
Submucosal Gland Cells
Beneath the surface epithelium of the trachea and large bronchi sit submucosal glands, small tubular structures that contribute a large share of airway mucus and fluid. These glands contain two main secretory cell types: mucous cells, which produce gel-forming mucins, and serous cells, which secrete a thinner, watery fluid rich in antimicrobial proteins.22PubMed Central. Fluid secretion by submucosal glands of the tracheobronchial airways Gene-expression profiling has confirmed that these two cell populations are highly specialized: mucous cells focus on mucin production, while serous cells concentrate on generating defensive molecules.23PubMed Central. Cellular and molecular architecture of submucosal glands in wild-type and cystic fibrosis pigs
Serous cells are particularly interesting because they express high levels of CFTR and secrete a remarkably diverse arsenal of antimicrobial compounds, including lysozyme, siderocalin (which starves bacteria of iron), and protease inhibitors like alpha-1-antitrypsin.24PubMed. Regulation of antiprotease and antimicrobial protein secretion by airway submucosal gland serous cells In cystic fibrosis, defective CFTR in serous cells impairs both the fluid secretion that flushes out gland contents and the antimicrobial cocktail itself, which helps explain why people with CF are so vulnerable to chronic lung infections.
Tight Junctions and the Physical Barrier
Regardless of which specific cell types are present at a given location, the epithelial sheet only works as a barrier if neighboring cells are sealed to one another. Tight junctions, protein complexes that encircle each cell near its top surface, control what can slip between cells. In the upper airway, the nasal epithelium is the first point of contact with inhaled antigens, and its tight junctions are a major part of innate defense.25PubMed Central. Regulation of tight junctions in upper airway epithelium Certain allergens, pollutants, and viral infections can loosen these junctions, increasing permeability and letting allergens or pathogens slip past the surface. This “leaky barrier” phenomenon is increasingly recognized as an early step in the development of allergic rhinitis and asthma.
The epithelium also produces antimicrobial peptides, collectins, and other molecules that neutralize pathogens directly on the airway surface. Together with the mucociliary escalator and the tight junctions, these chemical defenses form a three-part barrier system that operates continuously without any input from the adaptive immune system.26PubMed Central. Collectins and cationic antimicrobial peptides of the respiratory epithelia
Growing Airway Epithelium in the Lab
The diversity of cell types in the respiratory epithelium makes it one of the more challenging tissues to recreate outside the body, but doing so is essential for studying diseases like cystic fibrosis, asthma, and viral infections in a controlled setting. Researchers have developed protocols to coax human pluripotent stem cells into airway-like tissue by carefully timing the activation and deactivation of specific developmental signaling pathways. One approach uses cyclical modulation of Wnt signaling to guide stem cells through a lung progenitor stage and into functional proximal airway organoids, small three-dimensional structures containing multiple airway cell types.27Cell Stem Cell. Efficient Derivation of Functional Human Airway Epithelium from Pluripotent Stem Cells via Temporal Regulation of Wnt Signaling These organoids give researchers a way to test drugs, model genetic diseases, and study cell-cell interactions without relying on animal models. The progenitor cells at the heart of these cultures naturally tend toward proximal airway fates when Wnt signaling is dialed down, and toward distal (alveolar) fates when it stays high, mirroring the spatial patterning that occurs in the developing lung.
For patient-specific modeling, basal cells can be harvested from a nasal or bronchial brushing and expanded in culture, where they retain their ability to differentiate into ciliated and secretory cells. This has become a standard tool in cystic fibrosis research, allowing scientists to test how well new CFTR-modulator drugs restore function in tissue derived from an individual patient’s own cells. The practical payoff of understanding airway cell biology at this level is that it translates directly into better drug screening and, eventually, more personalized treatment strategies.