What type of cell is responsible for fighting off infections and foreign invaders in the body? 

White blood cells, also known as leukocytes, are responsible for fighting off infections and foreign invaders in the body. These cells are part of the immune system, which is the body's defense against harmful pathogens such as bacteria, viruses, and fungi. White blood cells have a crucial role in recognizing and responding to foreign invaders, by producing antibodies or attacking and destroying them directly. They are also involved in the removal of dead cells and other debris from the body, as well as the regulation of the immune response. Therefore, white blood cells play a critical role in maintaining the body's health and defending against infections, making them the type of cell responsible for fighting off infections and foreign invaders in the body.