While the human immune system comprises a vast network of specialized cells, the major primary classifications of white blood cells (leukocytes) driving immune defense include neutrophils, lymphocytes (which branch into T cells, B cells, and natural killer cells), monocytes/macrophages, and granulocytes (such as eosinophils and basophils). These diverse cellular populations work in concert to identify pathogens, neutralize foreign antigens, produce specialized antibodies, and coordinate complex inflammatory and adaptive immune responses throughout the body.