12.4A: Type I (Anaphylactic) Reactions
- Describe Type I hypersensitivity reactions
Type I hypersensitivity is also known as immediate or anaphylactic hypersensitivity. Anaphylaxis typically produces many different symptoms over minutes or hours. Symptoms typically include raised bumps on the skin (; hives), itchiness, red face or skin (flushing), or swollen lips.
Anaphylaxis can be caused by the body’s response to almost any foreign substance. Common triggers include venom from insect bites or stings, foods, and medication. Foods are the most common trigger in children and young adults. Medications and insect bites and stings are more common triggers in older adults. Less common causes include physical factors, biological agents (such as semen), latex, hormonal changes, food additives (e.g. monosodium glutamate (MSG) and food coloring), and medications that are applied to the skin (topical medications). Exercise or temperature (either hot or cold) may also trigger anaphylaxis by causing tissue cells known as mast cells to release chemicals that start the allergic reaction.
Anaphylaxis caused by exercise is often also linked to eating certain foods. If anaphylaxis occurs while a person is receiving anesthesia, the most common causes are certain medications that are given to produce paralysis (neuromuscular blocking agents), antibiotics, and latex. Many foods can trigger anaphylaxis, even when the food is eaten for the first time. In Western cultures, the most common causes are eating or being in contact with peanuts, wheat, tree nuts, shellfish, fish, milk, and eggs.
People with atopic diseases such as asthma, eczema, or allergic rhinitis have a high risk of anaphylaxis from food, latex, and radiocontrast agents. These people do not have a higher risk from injectable medications or stings. People who have disorders caused by too many mast cells in their tissues (mastocytosis) or who are wealthier are at increased risk. The longer the time since the last exposure to an agent that caused anaphylaxis, the lower the risk of a new reaction.
Anaphylaxis is a severe allergic reaction that starts suddenly and affects many body systems. It results from the release of inflammatory mediators and cytokines from mast cells and basophils. This release is typically associated with an immune system reaction, but may also be caused by damage to cells that are not related to an immune reaction. When anaphylaxis is caused by an immune response, immunoglobulin E (IgE) binds to the foreign material that starts the allergic reaction (the antigen ). The combination of IgE bound to the antigen activates FcεRI receptors on mast cells and basophils. The mast cells and basophils react by releasing inflammatory mediators such as histamine. These mediators increase the contraction of bronchial smooth muscles, cause blood vessels to widen (vasodilation), increase the leakage of fluid from blood vessels, and depress the actions of the heart muscle. There is also an immunologic mechanism that does not rely on IgE, but it is not known if this occurs in humans. When anaphylaxis is not caused by in immune response, the reaction is due to an agent that directly damages mast cells and basophils, causing them to release histamine and other substances that are usually associated with an allergic reaction (degranulation). Agents that can damage these cells include contrast medium for X-rays, opioids, temperature (hot or cold), and vibration.
Key Points
- Common triggers for anaphylaxis include venom from insect bites or stings, foods, and medication.
- People with atopic diseases such as asthma, eczema, or allergic rhinitis have a high risk of anaphylaxis from food, latex, and radiocontrast agents.
- Anaphylaxis is a severe allergic reaction that starts suddenly and affects many body systems due to the release of inflammatory mediators and cytokines from mast cells and basophils.
Key Terms
- anaphylaxis : A severe and rapid systemic allergic reaction to an allergen, causing a constriction of the trachea, preventing breathing; anaphylactic shock.
- hives : Itchy, swollen, red areas of the skin which can appear quickly in response to an allergen or due to other conditions.
- mast cells : A mast cell is a resident cell of several types of tissues and contains many granules rich in histamine and heparin. Mat cells play a role in allergy, anaphylaxis, wound healing and defense against pathogens.