barrier consists of type I pneumocytes, endothelial cells, and their fused
basal laminae. Pulmonary capillaries are sometimes in direct contact with
the alveolar wall, whereas in other locations, the alveolar wall and capillar-
ies are separated by cells and extracellular fibers. The areas of direct contact
are the location of gas exchange, whereas the other areas represent sites of
fluid exchange between the interstitium and air spaces. Macrophages are
present for the phagocytosis of debris and surfactant. The pores of Kohn
are connections from one alveolus to another, and macrophages travel
through these passageways. The pores normally equalize air pressure
between alveoli and can, in the disease state, provide collateral circulation
of air in the event that a bronchiole is blocked. However, they also provide
a passageway for the spread of bacteria.
191.The answer is e. (Kumar, pp 718–719.)IgM and IgG are serum anti-
bodies present in the alveolar fluid that activate complement by the classic
pathway. In that pathway, fixation of C1 to antibody combined with anti-
gen leads to activation of C3b, which binds to bacterial cell walls and
enhances opsonization. Neutrophils and macrophages have C3b receptors
that facilitate the opsonization. IgG also functions as an opsonin. The type II
pneumocytes resorb as well as secrete surfactant and surfactant-associated
proteins that have some antiviral and antibacterial function, but they do not
ingest microorganisms (answer a).CD8+T cells carry out cell-specific
killing(answer b).Mucociliary action is a critical component of the
immune function of the respiratory system, but clearance occurs in the
bronchioles → bronchi → trachea as part of the mucociliary apparatus.
Microorganisms are entrapped in mucus and then cilia propel them toward
the oropharynx. Microorganisms phagocytosed in the alveoli need to be
transported to the bronchioles in order to ride on the mucociliary escalator
(answer c).IgA functions to prevent attachment of microorganisms to the
epithelium, particularly in the upper respiratory tract (answer d).
Overall defense mechanisms of the respiratory system include nasal
clearance of material, which occurs through sneezing, whereas other material
may be swept into the nasopharynx and subsequently swallowed. The
mucociliary action within the trachea and bronchi is often called the mucocil-
iary, or tracheobronchial, escalator. At the distal end of the system, the alveo-
lar macrophages phagocytose foreign material and secrete and respond to an
array of cytokines. Neutrophils are attracted by chemokines and factors
derived from macrophages and other cells and phagocytose bacteria.
Respiratory System Answers 299