The primary component of the human immune system is white blood cells, also known as leukocytes. These cells are crucial for identifying and fighting off pathogens, such as bacteria, viruses, and fungi. Different types of white blood cells have specific roles; for instance, lymphocytes are essential for adaptive immunity, while phagocytes like macrophages engulf and digest foreign invaders. The immune system also includes organs such as the thymus, spleen, and bone marrow, which support the production and maturation of these vital cells. A well-functioning immune system is essential for maintaining health and preventing infection.