Which of the following substances triggers a hypersensitivity reaction?

a. Exchanging of gas: Alveoli are the primary sites for gas exchange in the lungs, where oxygen is absorbed into the blood and carbon dioxide is expelled.

b. Removing pathogens that are breathed in: This is a function of the respiratory tract's immune components, like macrophages in the alveoli, but not the primary function.

c. Containing mucus-producing goblet cells: Goblet cells are found in the respiratory tract but not in the alveoli.

d. Secreting surfactant to reduce surface tension: Alveolar cells do secrete surfactant, which is crucial for reducing surface tension and preventing alveolar collapse, but the primary function is gas exchange.

a. The plasma membrane is composed of a bilayer of protein molecules: This is incorrect. The plasma membrane is primarily composed of a bilayer of phospholipids, with proteins interspersed within this bilayer.

b. The plasma membrane is selectively permeable: This is correct. The plasma membrane regulates the entry and exit of substances, allowing certain molecules to pass while restricting others.

c. The plasma membrane does not participate in cellular activities: This is incorrect. The plasma membrane is actively involved in various cellular activities, including signaling, transport, and maintaining homeostasis.

d. The plasma membrane receptors are specialized lipids: This is incorrect. Receptors on the plasma membrane are typically proteins, not lipids, which are involved in signal transduction and cellular communication.