Which type of cells are a major component of the immune system?

White blood cells, also known as leukocytes, are the major component of the immune system. They play a crucial role in defending the body against both infectious disease and foreign invaders. There are various types of white blood cells, each with specific functions, including phagocytosis, producing antibodies, and systemic immune responses. They are essential for identifying and eliminating pathogens such as bacteria, viruses, and fungi.

In contrast, red blood cells are primarily responsible for transporting oxygen throughout the body, while platelets are involved in blood clotting. Stem cells, on the other hand, are undifferentiated cells that have the potential to develop into various cell types, including white blood cells, but they do not fulfill a direct role in immune response themselves. Thus, white blood cells are the critical players in the immune system, making this answer the most appropriate choice.