Ontario Grade 9 Science Practice Exam 2025 – Complete Study Resource

White blood cells are a critical component of the immune system and play a vital role in defending the body against infections and diseases. Their primary function is to identify and eliminate pathogens such as bacteria, viruses, and parasites. There are several types of white blood cells, each with specialized functions, including phagocytes that engulf and digest foreign invaders, and lymphocytes that produce antibodies targeting specific pathogens. This targeted response helps to keep the body healthy and free from illness.

The other options do not accurately reflect the role of white blood cells. While red blood cells are responsible for transporting oxygen throughout the body, some cells are involved in clotting blood, and energy is primarily provided through metabolic processes involving nutrients rather than blood cells. Thus, the correct answer emphasizes the defensive role of white blood cells in maintaining overall health.