Which blood cells are primarily responsible for fighting infection?

The blood cells primarily responsible for fighting infection are white blood cells, also called leukocytes. They are immune-system cells that identify, attack, and help coordinate defenses against microbes and other foreign substances.

White blood cells include several specialized types. Neutrophils are abundant early responders that engulf many bacteria and fungi. Lymphocytes include B cells, which produce antibodies, and T cells, which coordinate immune responses or destroy infected cells. Monocytes can develop into macrophages and engulf pathogens and damaged tissue, while natural killer cells target certain infected or abnormal cells.

All white blood cells originate from blood-forming stem cells in the bone marrow. Unlike mature red blood cells and platelets, they contain nuclei. They travel through the bloodstream and lymphatic system, and many can move into tissues where infection or inflammation is occurring.

The other listed options have different primary roles. Red blood cells transport oxygen, platelets help blood clot, and plasma is the liquid portion of blood that carries cells, proteins, nutrients, and waste. White blood cell counts can rise during infection, although abnormal counts can also reflect inflammation, medication effects, or blood disorders.