Final answer:
Leukocytes, or white blood cells, are primarily involved in the immune response and are the largest cells amongst the formed elements in the blood.
Step-by-step explanation:
The characteristic of all leukocytes compared to other formed elements in the blood is that they primarily involved in the immune response. Unlike erythrocytes (red blood cells), which are responsible for carrying oxygen, or platelets, which are involved in blood clotting, leukocytes are pivotal in defending the body against potentially harmful microorganisms or substances. Leukocytes, which are also known as white blood cells, are less numerous than erythrocytes and are larger than other formed elements. Additionally, they are the only complete cells with a nucleus among the formed elements. Therefore, the correct characteristic of leukocytes compared to other formed elements is that they are indeed the largest cells in the blood.