If your blood pressure falls suddenly and you feel faint, that may be a signal that you're having an allergic reaction, as the immune system rapidly causes hypotension. But this won't stay a subtle signal for long; it's associated with pretty major allergic reactions and you'll soon notice more obvious systems like difficulty breathing and hives.

Allergy-related coughing and post-nasal drip occur because of allergens in the air that enter your airways, and cause inflammation and an immune response, Dr. Elliott tells Bustle. It can be difficult to distinguish it from normal viral coughing, but it'll happen only in response to certain things (like pollen or smoke), may persist for weeks or months if the substance doesn't go away, and may not be accompanied by other cold-like symptoms.

The relationship between the immune system and your cardiovascular health is complicated, but heart palpitations are known to be a symptom of an allergic reaction, often one that comes on suddenly and is extreme in nature. If you notice your heart going extremely fast for no apparent reason, it could be the signal that your immune system is about to begin a serious reaction and you need to seek help.

This can be a side effect of the hypotension or low blood pressure induced by allergic reactions, but dizziness can also show up specifically in response to food allergies. It's not one of the more obvious symptoms that you're allergic to a particular food — like itchy mouth or swollen lips — but it's one to notice if you always feel light-headed after you consume something in particular.

The Red Cross notes that swelling of the extremities, the hands and feet, can be a sign of anaphylaxis, where the airways swell and constrict breathing in a life-threatening way. Swollen feet or hands accompanied by shallow breathing may be a medical emergency, and a signal for you to find the person's Epipen or call 911.

"Wrinkles on the nose and eyelids are atypical," says Dr. Elliott, but they're still possible symptoms of an allergic reaction. The nose wrinkle in people with allergic rhinitis is known as an "allergic crease" and runs horizontally across the nose, while new eyelid wrinkles can be caused by eyelid dermatitis, where the sensitive skin of the lid reacts poorly to an irritating substance.

If you have an allergy severe enough for your airways to start to constrict, one of the first symptoms may be an oddly hoarse voice. This is because air is struggling to pass out of the body, and causing odd modulations in vocal noise.

"Poor concentration," Dr. Elliott tells Bustle, is classed as an "atypical allergy symptom." In other words, brain fog might be a product of an immune system issue. Poor concentration has been reported as a widespread immune response to environmental allergens by the organization Allergy UK, particularly if they're paired with bad sleep quality that doesn't seem to be explained by other factors.

People who've survived anaphylactic shock often describe an emotional aspect to their experience; as they start to experience symptoms, they also start to feel panicky and anxious, in part because of lower oxygen supply to the brain. Panic can't cause allergic reactions, but it's not unknown for people who are having an allergic response to have panic attacks, as the experience can be quite scary.

If any of these subtle signs begins to happen, particularly if they're accompanied by other symptoms of an allergic reaction, don't ignore them. They could signal that your immune system wants you to pay attention.