On the skin, an acute type I allergic reaction causes hives, dermatitis, and itching, while chronically, the allergy may cause atopic dermatitis and eczema. In the respiratory tract, an acute allergic reaction causes coughing, nasal congestion, sneezing, throat tightness, and, chronically, asthma. It can also cause red itchy eyes. Acute allergic reactions in the gastrointestinal system start in the mouth with tingling, itching, a metallic taste, and swelling of the tongue and throat, followed by abdominal pain, muscle spasms, vomiting and diarrhea, chronically leading to a variety of gastrointestinal problems.
Any severe acute allergic reaction has the potential to be life threatening, causing anaphylaxis, a multi-organ reaction that can start with agitation, a feeling of "impending doom," pale skin due to low blood pressure, and/or a loss of consciousness (fainting). Anaphylaxis can be fatal without the rapid administration of an epinephrine (adrenaline) injection. Type I allergic reactions can be variable in severity, one time causing hives, the next time anaphylaxis.
Type I allergies can be in response to a variety of substances including but not limited to: foods, plants (pollens, weeds, grasses, etc), insect venoms, animal dander and saliva (such as cat and dog), dust mites, mold spores, occupational substances (latex), and drugs (such as penicillin). There can also be cross-reactions, where someone allergic to ragweed, for instance, may also react to melons (watermelon or cantaloupe) and banana. The most common food-related causes of severe anaphylactic reactions are peanuts, tree nuts such as walnuts, and shellfish.
Type IV delayed hypersensitivity reactions are most often skin reactions. A common example is the reaction to nickel in metal jewelry. Type IV hypersensitivity may cause redness, swelling, hardening of the skin, rash, and dermatitis at the exposure site hours to days after exposure.