Which of the following statements regarding leukocytes is TRUE?

A) Leukocytes have no nucleus.
B) Leukocytes play a role in oxygen transport.
C) Leukocytes are the largest of the formed elements.
D) Leukocytes are only fragments of cells.
E) There is only one kind of leukocyte.

Leukocytes, or white blood cells, are indeed among the largest of the formed elements in the blood, playing crucial roles in the immune system by defending the body against both infectious disease and foreign materials. Unlike red blood cells, which are primarily involved in oxygen transport, leukocytes are part of the body's immune response. They have a nucleus (contrary to option A), do not play a direct role in oxygen transport (contrary to option B), are complete cells rather than cell fragments (contrary to option D), and there are several types of leukocytes, each with different functions (contrary to option E).