What is an allergic reaction? the body's abnormal immune response to a foreign substance a mechanical injury caused by a foreign object a normal body function all of the above

The answer would be
"The body's abnormal immune response to a foreign substance"
because when a fairly new substance is rejected by the immune system the body goes into full defense mode, therefore, resulting in an Allergic Reaction. 

Answer psychoneuroimmunology;

Answer target heart rate;