Everybody has seen or been that guy at one point, who is standing out in literally freezing weather without a jacket of any sort, proclaiming that he isn’t cold. You chuckle as you point out that he must be cold since he’s doing such a poor job of hiding his shivering.
Of course, he just replies that hanging out in the cold makes you tougher, implying that you are some kind of warm weather weakling.
Machismo aside, standing outside in freezing temperatures without a jacket is a pretty decent display of toughness. But, does this kind of behavior actually make you tougher? Or, does it set you up for catching a cold like every mother has been saying since the dawn of time?
Fight or flight?
Norepinephrine is a hormone and neurotransmitter in a group of chemicals called catecholamines, sympathomimetic “fight-or-flight” hormones released by the adrenal glands in response to stress. It works in concert with epinephrine (commonly called adrenaline) to jump start the body’s response to potentially life-threatening situations. Their release into the body causes various things to happen, like increasing heart rate and mobilizing stored glycogen. Typically, conventional thinking will refer to these hormones’ immune suppressing capability.
The idea is, that perhaps the immune system isn’t as critical as glycogen mobilization while running away from a bear. I don’t think that simplistic explanation really makes sense. What happens if that bear is diseased and manages to get a good swipe at your back with his claws? There could be a host of pathogens scrambling to get inside of that wound.
This is where norepinephrine really starts to shine. You see, the actual effect catecholamines have on tissues during acute stress events is remarkably variable depending on the tissue and the situation. For instance, norepinephrine facilitates tissue repair  at wound sites and mobilization of leukocytes  to the location.
Rather than suppressing immune response, acute stresses actually seem to enhance it.
According to a study by the Army Research Institute of Environmental Medicine, says Dr. Belilovsky. “Researchers examined the immunological responses to cold exposure and found that acute cold exposure, such as going outside without a jacket, actually appears to activate the immune system.”  This occurs in part by increasing the levels of circulating norepinephrine, which works as a natural decongestant.
The key word here is “acute”. These stressors, like a wound or cold air exposure are sharp and temporary. The body senses the danger, activates hormones, and delivers a response. The immune system becomes markedly compromised when the hormonal response becomes dysregulated due to chronic stress exposure. Your body is just not equipped to deal with chronic stresses like nerve-wracking 12 hour work days, or prolonged severe cold exposure.
Beyond activity at the cellular level, there is a process by which one can face cold exposure and actually become more capable of attenuating the stress response. This is called hormesis, the physiological phenomenon where mild acute stresses can stimulate an organism or tissue to become stronger. 
It has become increasingly popular to utilize this mechanism, and is probably most often triggered on purpose by taking cold showers.  After getting over the initial shock response of the cold water, the body’s ability to regulate heat through thermogenesis will kick in and a few minutes later the water will feel more comfortable. Do this on a regular basis and you will probably experience a noticeable increase in cold water tolerance.
When does hormesis become hypothermia?
Of course, there is the evil twin version of this mechanism called hypothermia. One only needs to mention the sinking of the Titanic to invoke a fear a falling into frigid waters. What most people don’t realize is that the human body actually is hot enough to prevent heat loss in freezing water for up to 30 minutes.  What killed all of those unlucky passengers was that they were stranded in the freezing Atlantic ocean for far longer, and lost core body heat from swimming. Tragically, the need to tread water causes the body to lose heat faster, and the numbing of limbs eventually makes it impossible to stay above the water.
In a basic way, this is another illustration of the differences between acute and long term stressors, and how the body can cope up to a point. A cold shower is an acute stress, typically only a few minutes of cold water exposure. It’s just enough to get the body excited and activate it’s catecholamine response. Engaging in this type of controlled and intermittent training will make you tougher.
Slogging your way through a marathon during a blizzard, or immersing yourself in icy water for hours on end will do the opposite. If you don’t die outright from heat loss, your body’s organs and peripheral systems will be compromised just from the shear need to keep core functionality like your brain operational.
So, will cold exposure make you stronger or weaker?
If you plan on doing some hiking or trail running this winter, be sure to dress appropriately. I wouldn’t want your mom to get worried.