What does the immune system protect the body against?
changes of the internal body temperature
changing environmental conditions
changes caused by harmful pathogens
changes in the ph of the blood

The correct answer is changes caused by harmful pathogens.

The immune system of our body is known to protect the body from the foreign antigens, which can cause the harm to the body of the organism. The immune system kills the pathogen, which invades our body and provide the protection against the changes, which can caused by the pathogens.

The homeostasis mechanism of the body is responsible for maintenance of the stable body environment. The changes of the internal body temperature, changing environmental conditions, changes in the pH of the blood etc are controlled by homeostasis mechanism of the body.

the answer that best describes the relative refractory period in the neuron is the resting membrane potential is restored, but concentrations of sodium and potassium are not restored to their original state.

the refractory period is the time during which another stimulus given to the neuron will not lead to a second potential. thus, because na+ channels are inactivated during this time, additional depolarizing stimuli do not lead to new action potentials.  

read more on -