Which statement about connective tissue proper cells is TRUE?

Mast cells play a crucial role in the immune response and are known for their ability to produce and store two important substances: heparin and histamine. Heparin is an anticoagulant that helps prevent blood clotting and ensure adequate blood flow, while histamine is involved in the inflammatory response, causing blood vessels to expand and enabling other immune cells to access the site of injury or infection. This dual role in producing both heparin and histamine makes mast cells integral to the function of connective tissue, particularly in mediating allergic reactions and responding to pathogens.

The other statements misrepresent the roles and identities of various immune cells. For example, plasma cells are not derived from neutrophils; rather, they are differentiated B cells that produce antibodies. Furthermore, while neutrophils are common white blood cells, they are predominantly associated with acute responses and are not the primary cells of connective tissue proper. Lastly, mast cells are indeed known to produce heparin, so the confirmation of their ability to produce both heparin and histamine solidifies the correctness of the chosen statement.