I think it's useful to visualise the graph when considering this.
In alarm you haven't really assessed the stressor yet, so you have things like the rate of blood being pumped around your body reducing - because that way if you get cut you'll have less blood loss.
Counteralarm you get FFF which counteracts that and energises you, ready to act in response to anything. As far as I know, there are no symptoms associated with this.
In resistance cortisol is having a significant impact. It frees up more energy stores for you to use but it also suppresses components of the immune system.
So in this stage you become more susceptible to colds. You're effective against the stressor that prompted this, but not much else.
Overtime, this drains your biological and psychological resources. Once you hit exhaustion, not only are you vulnerable to other things making you sick, you lack the resources to continue standard functioning. You're not effective against anything.
Hope this helps