This review article presents fundamental mechanisms of the local mucosal immunity in selected
regions of the respiratory tract in healthy birds and in some pathological conditions. The respiratory
system, whose mucosa come into direct contact with microorganisms contaminating inhaled air, has
some associated structures, such as Harderian gland (HG), conjunctive-associated lymphoid tissue
(CALT) and paranasal glands (PG), whose participation in local mechanisms of the mucosal immunity
has been corroborated by numerous scientific studies.
The nasal mucosa, with structured clusters of lymphoid tissue (NALT – nasal-associated lymphoid
tissue) is the first to come into contact with microorganisms which contaminate inhaled air. Lymphoid
nodules, made up of B cells with frequently developed germinal centres (GC), surrounded by a coat of
CD4+ cells, are the major NALT structures in chickens, whereas CD8+ cells are situated in the epithelium
and in the lamina propria of the nasal cavity mucosa.
Studies into respiratory system infections (e.g. Mycoplasma gallisepticum) have shown the reactivity
of the tracheal mucosa to infection, despite a lack of essential lymphoid tissue.
Bronchus-associated lymphoid tissue (BALT) takes part in bronchial immune processes and its
structure, topography and ability to perform defensive function in birds is largely age-dependent.
Mature BALT is covered by a delicate layer of epithelial cells, called follicle-associated epithelium
(FAE). Germinal centres (GC), surrounded by CD4+ cells are developed in most mature BALT
nodules, while CD8+ lymphocytes are dispersed among lymphoid nodules and in the epithelium, and
they are rarely present in GC.
Macrophages make up the first line of defence mechanisms through which the host rapidly responds
to microorganisms and their products in the respiratory mucosal system. Another very important
element are polymorphonuclear cells, with heterophils being the most important of them. Phagocytic
cells obtained from lung lavages in birds are referred to as FARM (free avian respiratory macrophages).
Their number in chickens and turkeys is estimated to be 20 times lower than that in mice and rats,
which indicates a deficit in the first-line of defence in the birds’ respiratory system.
There are numerous B cells and antibody secreting cells (ASC) present throughout the respiratory
system in birds. Their role comes down to perform antigen-specific protection by producing antibodies
(IgM, IgY or IgA class) as a result of contact with pathogenic factors.
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