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The purpose of physiological respiration is to supply cells with __________ from the environment. 

True or False: Humans have a closed circulatory system. 

______________ carry messages between neurons and other cells. 

Compare and contrast the first and second lines of defense. List two components of each line of defense. 

_________ are cells with specialized proteins called actin and myosin. 

 Upon activation, B-cells will divide to form _________. 

True or False: The testes are suspended outside of the abdominal cavity because proper sperm development occurs below normal body temperature. 

All B and T cells begin development in ___________. 

True or False:  Stem cells in the lymph nodes divide to form all of the blood cells. 

Which type of T-cell is responsible for activating B-cells and other T-cells?