Contact of pathogens with the innate immune system will most frequently occur at epithelia, and the biology of the airway epithelium is of considerable importance in asthma. Airway epithelia express a range of innate immune receptors, allowing them to function as a line of first response to pathogens: their ability to detect and respond to pathogens must clearly be substantial, given that they form the main target for most respiratory viruses.
There are also potentially close relationships between epithelial cells and other cells of the innate immune system such as DCs and macrophages.
Cooperative networks that regulate airway inflammation are discussed in more detail below. Interestingly, defective responses to respiratory viruses are evident in epithelial cells from asthmatics, which may be relevant in the pathology of asthma exacerbations, and phenotypic differences in epithelia between asthmatics and normal subjects have been demonstrated.
Effective barrier functions result from a combination of humoral and cellular mechanisms, as classically illustrated in the respiratory tract, where mucus and lining fluid serve to neutralize and clear pathogens, and epithelial cells act to impede pathogen access to the body, as well as providing a first line of response to pathogens.
