Where in the body are white blood cells primarily found?

White blood cells, also known as leukocytes, are primarily found in the lymphatic system, which includes key components such as lymph nodes and the thymus gland. The lymph nodes serve as filters for pathogens and are critical in the immune response, allowing white blood cells to recognize and combat infections. The thymus gland is also crucial as it is the site where T cells, a type of white blood cell, mature.

This makes option B the most accurate answer, as it highlights the primary locations in the body where white blood cells are concentrated and play critical roles in immune function. Other options mention organs that have different primary functions, which are not the main sites for white blood cell activity and production.