Platelets are cell fragments that are released by what type of cell?

A. Tunica media: Middle layer made of smooth muscle and elastic fibers; not the innermost layer.
B. Tunica externa: Outermost connective-tissue layer (adventitia) that contains collagen and sometimes vasa vasorum; not the innermost.
C. Endothelium: The simple squamous cell lining that forms the innermost surface (part of the tunica intima).
D. Vasa vasorum: Small blood vessels that supply the walls of large vessels, located in the outer layers; not the innermost.

A. To produce antibody molecules: plasma cells are differentiated B cells specialized for synthesizing and secreting antibodies.
B. To directly attack cells displaying antigens: cytotoxic T lymphocytes (and NK cells) directly attack antigen-bearing cells; plasma cells primarily secrete antibodies.
C. To increase the volume of the blood plasma: plasma cell activity does not change plasma volume; they produce proteins (antibodies), not fluid volume.
D. To produce interferon: many cell types secrete interferons in response to viral infection (e.g., infected cells, immune cells), but plasma cells are dedicated antibody producers.