What is the primary component of blood involved in immunity?

The primary component of blood involved in immunity is leukocytes, or white blood cells. These cells play a crucial role in the immune response by identifying and attacking pathogens such as bacteria, viruses, and other foreign invaders. Different types of leukocytes have specialized functions; for example, lymphocytes are responsible for the adaptive immune response, while phagocytes, like neutrophils and macrophages, engulf and destroy pathogens.

Erythrocytes, or red blood cells, are primarily responsible for oxygen transport in the body and do not play a direct role in immunity. Platelets are involved in blood clotting and wound healing, again not directly related to immune functions. Plasma proteins, while they include antibodies that aid in immune responses, are not considered the primary cellular component responsible for immune activity compared to leukocytes. Thus, the correct choice reflects the integral role that leukocytes have in maintaining immune defense mechanisms.