White blood cells primarily contribute to which bodily defense?

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White blood cells are central to the body's defense system—immunity. They patrol the bloodstream and lymphatic system, recognize foreign invaders, and respond in multiple ways. Some WBCs, like neutrophils and macrophages, act as phagocytes that engulf and destroy bacteria and debris. Others, such as lymphocytes, coordinate the adaptive immune response; B cells produce antibodies that target specific pathogens, while T cells help orchestrate the attack and destroy infected cells. They also release signaling molecules that recruit more immune cells and promote inflammation to contain threats. Because of these roles, white blood cells are primarily involved in defending against infections, not in digestion, gas exchange, or circulating blood components; digestion relies on enzymes and acids, respiration on oxygen transfer often carried by red blood cells, and circulation mainly concerns moving blood, with immune cells dispersed throughout. Thus, their main contribution is immunity.