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Bronchopulmonary Tract Immunity

For medical students2 min readUpdated 2026-10-10

Protection of the respiratory tract against infections relies on the close interaction between mucosal epithelial cells and the immune system. The epithelial barrier not only physically blocks pathogens but also actively participates in antigen recognition, instantly transmitting signals to the cellular arm of the immune system.

Basis of defenseCooperation between mucosal epithelial cells and the immune system.
M-cellsCapable of binding antigens without any prior stimulation.
T-lymphocytesCD8+ lymphocytes predominate within the epithelial layer.
MacrophagesPrimarily represented by resident, functionally inactive forms.

Functions of the Respiratory Epithelium

The lining tissues of the respiratory tract perform two basic functions: barrier (physical limitation of pathogen penetration) and secretory.

A special role in the structure of the epithelial sheet is played by specialized M-cells. Their main distinguishing feature is the ability to capture, bind, and present foreign antigens to the immune system completely autonomously, without the need for prior stimulation. In addition, M-cells actively secrete signaling molecules (cytokines), triggering a cascade of immune reactions.

Dendritic Cells and Their Migration

Dendritic cells act as a crucial link within the mucosa. Their movement and localization in the tissues of the bronchopulmonary tract are strictly regulated:

The functional activity of dendritic cells is not constant. It increases manifold against the background of an inflammatory process, as well as upon direct contact with waste products or disintegration products of microorganisms.

Effector Cells: T-Lymphocytes and Phagocytes

The cellular composition responsible for pathogen destruction is distributed across different layers of the respiratory tract:

  1. T-lymphocytes: Cytotoxic CD8+ cells concentrate predominantly directly within the epithelial layer of the mucosa.
  2. Phagocytes: Neutrophils and macrophages are distributed in various layers of the bronchial walls and alveolar septa.

Important clarification regarding macrophages: the vast majority of macrophages in the bronchopulmonary tract are resident cells that previously differentiated from circulating blood monocytes. Under normal conditions (prior to infection), only a very small fraction of them is in an active state; the bulk remains resting.

Mnemonic

To remember the key players in pulmonary immunity, use the acronym MDTP: M-cells (epithelium), Dendritic cells, T-lymphocytes (CD8+), Phagocytes (neutrophils and macrophages).

Frequently asked questions

Which immunoglobulin classes provide humoral immunity in the respiratory mucosa?

Humoral immunity of the respiratory mucosa is provided by immunoglobulins of classes A, G, and M.

  • Immunoglobulin A (IgA) — the most important effector factor of mucosal lymphoid tissues, formed from dimeric molecules. Secretory IgA protects the airways from microbes.
  • Immunoglobulin G (IgG) — its synthesis predominates in mucous membranes free of microflora, such as the lower respiratory tract.
  • Immunoglobulin M (IgM) — produced in the early stages of the immune response on days 4–7, but does not play a major role in mucosal defense.
What antimicrobial peptides are secreted by the epithelium of the bronchopulmonary tract?

Antimicrobial peptides contained in the secretion of the respiratory mucosa include β-defensins. They are part of the mucus produced by the nasal glands of the lamina propria of the respiratory region.

Through what structures is mucociliary clearance carried out in the respiratory tract?

Mucociliary clearance in the respiratory tract is carried out through the interaction of the ciliary apparatus and secretory structures of the epithelium.

  • Ciliated cells — contain up to 250 cilia on the apical pole, consisting of an axoneme (microtubules) and dynein arms. The beating of the cilia moves mucus with foreign particles toward the pharynx.
  • Goblet cells and Club cells — located in the surface epithelium and secrete mucus.
  • Bronchial glands — also provide the production of mucosal secretion necessary for clearance function.
What receptors do alveolar macrophages use to recognize pathogen-associated molecular patterns?

To recognize pathogen-associated molecular patterns (PAMPs), macrophages use several groups of membrane receptors.

  • Toll-like receptors (TLRs) — macrophages express TLR-1, TLR-2, TLR-3, TLR-4, TLR-5, TLR-6, TLR-7, TLR-8, TLR-9, and TLR-10.
  • C-type lectin receptors (CL receptors) — include the macrophage mannose receptor (group VI) and dectin-1 (group V).
  • Scavenger receptors.
  • Integrins.

These receptors allow macrophages to recognize various components of microorganisms; for example, TLR2, TLR4, and dectin-1 are involved in recognizing the fungal cell wall.

Which specific T-lymphocytes localize in the respiratory epithelium?

CD8+ T-lymphocytes are predominantly present within the epithelial layer of the bronchopulmonary tract.

What is the unique function of M-cells?

They are capable of binding and presenting antigens without prior stimulation and independently secrete cytokines to activate other cells.

How do dendritic cells migrate into the mucosa?

Their directed migration is controlled by chemokines and adhesion molecules (primarily $\beta_2$-integrins).

What state are most macrophages in the lungs in?

Most of them are resident cells (derived from monocytes), and only a very small fraction of them is in a functionally active state.

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