There are two primary processes that contribute to sensitized skin. While visibly red, irritated skin may be the end result of two different types of inflammation, immunogenic and neurogenic, one can also have these types of inflammation without visible signs (ie redness) associated with it. The inflammatory process is still occurring but it is invisible to the eye and may only result in stinging, burning, itching. In this case the skin is sensitized. If not controlled it may lead to visible inflammation associated with redness and the typical symptoms. 

• IMMUNOGENIC INFLAMMATION is defined as the body, in this case, the skin, responding to an introduced irritant (pollen, bacteria and artificial fragrances are common triggers) with an immune-system response. In other words, the skin attacks the intruding force as though it is a pathogen which must be eradicated from the body, much the way your body would fight off a rhinovirus which causes the common cold, etc. However, in the case of sensitization, pain, redness and swelling are common responses. 

• NEUROGENIC INFLAMMATION originates in the nerves, and nervous system. Chemicals and pollutants in the environment stimulate receptors in the skin to trigger inflammation resulting in the release of substances called neuropeptides that trigger the inflammatory response. Under normal conditions, these substances play a highly useful role in tissue repair, i.e. wound healing. However, neuropeptides also are active in triggering and aggravating sensitization as well as,a variety of painful inflammatory conditions such as urticaria (itching), psoriasis, atopic dermatitis and rosacea. Combine either or both of these factors above with a compromised barrier lipid layer and you have the perfect conditions to have inflammed overly reactive skin. 

What triggers either or both of these two complex processes may be in fact a multitude of triggers. The simplest answer is “stress”—defined any number of ways:, mental/emotional “life” stress – challenges with family, work, finances, other social or relationship-oriented interactions • sheer physical stress caused by fatigue, systemic dehydration or malnutrition, or literal metabolic exhaustion or overexertion—from extended travel, rigorous activity or athletic training, or recovery from illness • stress of an environmental variety, such as the immune system being challenged by toxins in the air, water, or other physical conditions  • Smoking, which dehydrates the skin by depleting barrier lipids and affects collagen synthesis and cell metabolism • Exposure to pollution, Excessive exfoliation leading to over-processed skin, especially combinations of common procedures and products such as microdermabrasion, glycolic peels, retinoids, etc.  Excessive washing, especially with alkaline products like conventional bar-soap, and especially showering/bathing in too-hot water. Topical products and makeup containing artificial fragrances, colorants (D&C or FD&C colors) and SD alcohol. Alcohol and caffeine consumption, can dehydrate the tissues and in some clients dilate capillaries, resulting in flushing and redness.  Inadequate UV protection; oftentimes chemical sunscreens (non titanium dioxide or zinc oxide sunscreens) can trigger sensitization.  Nutritional choices : check into habits such as diet pills (see caffeine comment, above) and lowfat diets which may deplete lipid strength. Also, eating spicy foods has been linked to increased skin sensitivity, possibly due to nerve activity in the skin. Stress and sleep-deprivation .

Extracted from dermalogica.com