The primary role of white blood cells (leukocytes) in the immune system is to defend the body against infections and foreign invaders. They play a crucial role in identifying, attacking, and eliminating pathogens like bacteria, viruses, and parasites. Different types of white blood cells, such as lymphocytes and phagocytes, have specialized functions in recognizing and responding to these threats. A healthy immune system relies on proper white blood cell function to protect the body from diseases and maintain overall health.