Which statement describes many of the structures involved in the first line of defense of the immune system? They have contact w
ith the external environment. They are internal and contact the pathogen through blood. They produce B-cells in response to a pathogen. They produce T-cells in response to a pathogen.
Darwin called the ability of an organism to survive and reproduce in its environment fitness. Some organisms were fit to do all of these things, whereas others were not.
