The body's first reaction to a stressor, in the is a temporary state of shock during which resistance to illness and stress falls below normal limits.

Answer;
Alarm stage
The body's first reaction to a stressor, in the alarm stage, is a temporary state of shock during which resistance to illness and stress falls below normal limits.

Explanation; 
 Alarm stage is the first stage in the general adaptation syndrome; others are the resistance stage and the exhaustion stage;
During the alarm stage; The body's first reaction to a stressor is a temporary state of shock during which resistance to illness and stress falls below normal limits. In an attempt to code, the body releases hormones that, in a short time, adversely affect the functioning of the immune system. During this time the body is susceptible to infections from illness or injury.