The primary function of white blood cells (leukocytes) in the human body is to defend against infections and diseases. White blood cells are an integral part of the immune system, seeking out and destroying pathogens, such as bacteria, viruses, and fungi. There are different types of white blood cells, including lymphocytes, neutrophils, and monocytes, each with specific roles in the immune response. They help identify and remember invaders, producing antibodies and coordinating the body's defense mechanisms, thereby maintaining health and preventing illness.