Functions of the respiratory epithelium
The surface tissues of the respiratory tract fulfill two fundamental tasks: barrier function (physical restriction of pathogen entry) and secretory function.
Specialized M cells play a unique role in the structure of the epithelial layer. 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. Furthermore, M cells actively secrete signaling molecules (cytokines), triggering a cascade of immune reactions.
Dendritic cells and their migration
Dendritic cells serve as a crucial connecting link in the mucosa. Their movement and localization within the tissues of the bronchopulmonary tract are strictly regulated:
- Directed movement is driven by the action of specific chemokines.
- Fixation and interaction occur with the participation of cell adhesion molecules, specifically $\beta_2$-integrins.
The functional activity of dendritic cells is not constant. It increases manifold during the development of an inflammatory process, as well as upon direct contact with microbial waste products or decay products.
Effector cells: T lymphocytes and phagocytes
The cellular composition responsible for pathogen destruction is distributed across different layers of the respiratory tract:
- T lymphocytes: Cytotoxic CD8+ cells are concentrated predominantly directly within the epithelial layer of the mucosa.
- Phagocytes: Neutrophils and macrophages are distributed across 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 are in an active state; the bulk remain quiescent.
Briefly: answers to topical questions
Which classes of immunoglobulins 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) is 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) — is 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 respiratory mucosal secretion include beta-defensins. They are part of the mucus produced by the nasal glands of the lamina propria of the respiratory region.
What structures drive mucociliary clearance in the respiratory tract?
Mucociliary clearance in the respiratory tract is carried out through the interaction of the ciliary apparatus and the secretory structures of the epithelium.
- Ciliated cells — contain up to 250 cilia at 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 — are located in the surface epithelium and secrete mucus.
- Bronchial glands — also ensure the production of the mucous secretion necessary for clearance to 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 — garbage-collector receptors.
- Integrins.
These receptors allow macrophages to recognize various microbial components; for example, TLR2, TLR4, and dectin-1 are involved in recognizing the fungal cell wall.
To memorize the key players in pulmonary immunity, use the acronym MDTF: M cells (epithelium), Dendritic cells, T lymphocytes (CD8+), Fhagocytes (neutrophils and macrophages) [Note: Phagocytes in Russian].
Class and exam questions
Which specific T lymphocytes are localized in the respiratory epithelium?
Predominantly CD8+ T lymphocytes are 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.
What drives dendritic cells to migrate into the mucosa?
Their directed migration is controlled by chemokines and adhesion molecules (primarily $\beta_2$-integrins).
What is the functional state of most macrophages in the lungs?
Most of them are resident cells (derived from monocytes), and only a very small fraction of them are in a functionally active state.
What else to study on this topic
- Mechanisms of secretory activity of the respiratory epithelium
- The process of antigen presentation by specialized M cells
- The role of $\beta_2$-integrins and chemokines in dendritic cell migration
- Stages of monocyte differentiation into resident lung macrophages
- Mechanisms of phagocytic activation during bronchial inflammation
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