Which type of white blood cell is primarily involved in the immune response?

Neutrophils are a type of white blood cell that play a critical role in the immune response, especially in the defense against bacterial infections. They are the most abundant type of granulocytes and are often the first responders to sites of infection or inflammation. When pathogens invade the body, neutrophils migrate to the affected area, where they can engulf and destroy the invading microbes through a process called phagocytosis. They also release various enzymes and antimicrobial substances to help neutralize threats.

Erythrocytes, or red blood cells, are primarily responsible for the transport of oxygen and carbon dioxide in the bloodstream, and they do not play a direct role in the immune response. Platelets are involved in blood clotting and wound healing, rather than directly fighting infections. Bacteria are pathogens that can trigger an immune response but are not types of white blood cells themselves. Therefore, neutrophils are clearly the key players in the immune response among the listed options.