Where does the circulating lymph return to cardiovascular circulation?

a. Pathogen: A pathogen is a disease-causing organism like bacteria, virus, or parasite. It can trigger an immune response, but not necessarily a hypersensitivity reaction.

b. Allergen: An allergen is a foreign substance (like pollen, dust mites, or food) that can trigger an immune response in an already sensitized individual. This response can lead to allergy symptoms like hay fever or anaphylaxis

c. Immunogen: An immunogen is any substance that can trigger an immune response, but not necessarily a hypersensitivity reaction. It can be a pathogen, allergen, or other foreign substance.

d. Fibrinogen: Fibrinogen is a blood protein involved in blood clotting, not allergic reactions.

a. Secretes fluid to reduce friction between layers: This is correct. The pleural membranes secrete pleural fluid, which reduces friction between the visceral and parietal layers during breathing.

b. Secretes a substance that reduces surface tension of alveoli: This describes the function of surfactant, which is secreted by alveolar cells, not pleural membranes.

c. Secretes a substance to keep epithelial cells wet: This is a general description that doesn't specifically apply to pleural membranes.

d. Secretes fluid involved in protection from infections: While pleural fluid may have some protective roles, its primary function is to reduce friction, not directly to protect from infections.