What happens when you have an allergic reaction?
Allergists recognize four types of allergic reactions: Type I or anaphylactic reactions, type II or cytotoxic reactions, type III or immunocomplex reactions and type IV or cell-mediated reactions.
Allergic reactions are exaggerated sensitivities (hypersensitive reactions) that occur when your immune system responds abnormally to common substances such as pollen, dust and certain foods.
These substances, called allergens, are harmless in a majority of people. But for those who are allergic, they can cause reactions upon skin contact or when they are breathed, swallowed or injected.
Allergic reactions are quite common and may happen seconds to hours after contact with the allergen. Some reactions may take more than 24 hours to appear. Though many allergic reactions are mild, others may be dangerous or life-threatening. They may be localized, involving a small part of the body or may affect a large area or the whole body.
Certain metal jewellery or certain cosmetics may cause skin rash in some, for example. Others sneeze uncontrollably on exposure to dust or pollen.
An allergic reaction begins when you touch, inhale or swallow an allergen. In response to this trigger, the body starts making a type of protein called IgE or immunoglobulin E.
IgE molecules bind with the allergen molecules in an antigen-antibody reaction. This attachment of the antigen and antibody leads to the release of some chemicals (such as histamine) in the body. These chemicals cause the inflammatory symptoms of allergic reactions such as rashes, itching and sneezing.
What are the four types of allergic reactions?
Allergens can cause allergic reactions upon skin contact, when they are breathed, swallowed or injected.
Two British immunologists, Coombs and Gell, have classified allergic reactions into four types, Type I, II, III and IV.
Type I, II and III allergic reactions are called immediate types of allergic reactions because they occur within twenty-four hours of exposure to the allergen. Type IV reactions typically occur after 24 hours of exposure and are called delayed allergic reactions.
Type I or anaphylactic reactions: Type I reactions are mediated by proteins called IgE antibodies produced by the immune system. These are produced in response to the allergens such as pollen, animal dander or dust mites, or even certain foods. This causes the release of histamine and other chemicals causing inflammation and swelling. Examples of type I allergic reactions include
- bronchial asthma,
- allergic rhinitis,
- allergic dermatitis,
- food allergies,
- allergic conjunctivitis (eye inflammation) and
- anaphylaxis (allergic shock).
Anaphylaxis is the most severe form and is a medical emergency because it can lead to a sudden, life-threatening respiratory failure. People with anaphylaxis have extreme difficulty in breathing, swelling, low blood pressure, bluish skin and shock.
Type II or cytotoxic reactions: This type of allergic reaction is mediated by proteins called IgG and IgM antibodies. The antibodies involved in type II reaction damage cells by activating a component of immunity called the complement system. Type II allergic reactions can be seen in certain conditions like
- autoimmune hemolytic anemia,
- immune thrombocytopenia and
- autoimmune neutropenia.
Type III or immunocomplex reactions: Type III reactions are also mediated by proteins i.e. IgM and IgG antibodies. These antibodies react with the allergen to form immunocomplexes (antigen-antibody complexes). These complexes are responsible for the reaction. Type III allergic reactions can be seen in
- lupus,
- serum sickness and
- Arthus reaction.
Type IV or cell-mediated reactions: Type IV allergic reactions are also called the delayed type of hypersensitivity or allergic reactions as they occur after at least 24 hours of exposure to the allergen. These reactions typically take 48-72 hours or longer to appear after contact with the allergen. Many long-term infectious diseases, such as tuberculosis and fungal infections, show cell-mediated reactions. Certain skin sensitivity reactions esp. to metals may also belong to this type.
What are the triggers of allergic reactions?
Pollen, foods, dust mites, animal or pet dander, bee or wasp stings, and meidcations are a few examples of allergens that can trigger allergies.
Substances that trigger an allergic reaction are called allergens. They include:
- Dust mites
- Animal/pet dander
- Pollen
- Bee/wasp stings
- Certain medications such as penicillin and penicillin-based antibiotics
- Certain foods such as soy, eggs, peanuts, tree nuts, fish, shellfish, milk and wheat
- Mold
- Latex
- Certain metals
- Some plants