Respiratory Tract Immunity and Mucosal Defense

The respiratory tract presents a vast, thin epithelial surface to inhaled air, microbes, and allergens, and it must defend itself while keeping gas exchange unobstructed. Respiratory mucosal immunity combines the airway epithelial barrier, mucociliary clearance, and innate and adaptive immune responses to repel pathogens at the point of entry without provoking damaging inflammation.

Definition

Respiratory tract immunity is the mucosal immune defence of the airways and lungs, integrating the epithelial barrier and mucociliary clearance with innate sensing and adaptive responses to protect the respiratory surface against inhaled pathogens while limiting immunopathology.

Scope

This topic covers the airway and lung mucosal barrier, the rapid local innate defences against respiratory pathogens, and the regulation of type 2 immunity at the airway surface. It is a reference entry within mucosal immunology and is not clinical guidance for respiratory disease.

Core questions

• How does the airway epithelium defend against inhaled pathogens at the point of entry?
• What innate mechanisms provide the earliest respiratory defence?
• How is type 2 immunity at the airway controlled, and when does it become harmful?
• How does respiratory immunity compare with immunity at other mucosal sites?

Key concepts

• Airway epithelial barrier
• Mucociliary clearance
• Antimicrobial peptides and surfactant
• Innate antiviral defence (interferons)
• Alveolar macrophages
• Type 2 immunity and epithelial alarmins
• Bronchus-associated lymphoid tissue (BALT)

Mechanisms

The respiratory epithelium forms a barrier covered by mucus and cleared by coordinated ciliary beating, removing trapped particles and microbes. Epithelial cells and resident innate cells, including alveolar macrophages, sense pathogens and mount early local defences such as interferon responses against respiratory viruses. Epithelial cells also act as sentinels for type 2 immunity, releasing alarmins that can drive allergic and antiparasite responses, which must be tightly regulated to avoid airway inflammation. Adaptive immunity, including local antibody and T-cell responses, supplements these defences. As at other mucosal sites, protection is balanced against the need to limit immunopathology in a delicate gas-exchange tissue.

Clinical relevance

Respiratory mucosal immunity underlies defence against respiratory infections and shapes diseases such as asthma and allergic airway inflammation, and it is the target of mucosal and intranasal vaccination strategies. This entry presents the underlying biology for reference and is not a basis for individual diagnosis or treatment.

History

Respiratory immunology developed from the recognition that the airway is an active immune barrier rather than a passive conduit, with study of mucociliary clearance and alveolar macrophages giving way to detailed accounts of early local innate defences and of how barrier epithelial cells initiate and regulate type 2 immunity.

Key figures

• Akiko Iwasaki
• Bart Lambrecht
• Hamida Hammad

Seminal works

• iwasaki-2016
• hammad-2015

Frequently asked questions

What is mucociliary clearance?

It is the airway's mechanical defence in which a mucus layer traps inhaled particles and microbes and coordinated ciliary beating sweeps the mucus upward out of the lungs, removing trapped material before it can establish infection.

Why is type 2 immunity important at the airway?

Airway epithelial cells help initiate type 2 immune responses against parasites and allergens; when this response is dysregulated it contributes to allergic airway inflammation and asthma, so its control is central to respiratory mucosal immunity.