Allergies are over-reactions of the immune system in certain individuals to seemingly and generally harmless foreign proteins and substances.
When do allergies occur?
Allergies usually do not occur at the first exposure. When a person is exposed to an allergen for the first time, the body develops molecules called antibodies against the invading proteins. This is called an immune response.
When exposed to the allergen again the immune system produces large amounts of antibodies that lead to break down of mast cells that contain chemicals like histamine. This leads to the features of allergies.
This process is known as sensitisation. Sensitization may take days to years. Sometimes sensitization develops as the person affected shows symptoms but never fully develops the allergy to the allergen.
Symptoms of allergy
Symptoms of allergy include:
- shortness of breath
- runny nose and eyes
- pain over the sinuses (at the bridge of the nose, near the eyes, over cheeks and at the forehead)
- skin rashes (nettle rashes or hives)
- swelling of the lips or face
- itching eyes, ears, lips, throat and roof of the mouth
- abdominal cramps and diarrhoea
When allergic reaction is life threatening of severe it is termed anaphylaxis or anaphylactic shock. Anaphylaxis involves the whole body.
- swelling of the throat and mouth and clogging up of airways leading to difficulty breathing, difficulty in speaking or swallowing
- rash and itching elsewhere in the body
- weakness and collapse often with unconsciousness due to sudden fall in blood pressure
Anaphylaxis requires urgent emergency management.
Classification of allergies
Types of allergy are classified to denote cause, severity and possible management and prevention. These include –
- Type I hypersensitivity
This is also known as immediate or anaphylactic-type reactions. This may be caused due to pollen, foods and drugs and insect stings.
- Type II hypersensitivity
This involves specific antibodies called the Immunoglobulin G (IgG) and IgM. There is binding to and destroying the cell the antibody is bound on.
This type of reaction is seen after an organ transplant when the body refuses to see the transplanted organ as its own.
- Type III hypersensitivity
This is an Immune complex-mediated reaction. The immune complex is the bound form of an antibody and an antigen.
This leads to a cascade of reactions in the body which goes on to destroy local tissues. Examples of this condition include glomerulonephritis and systemic lupus erythematous (lupus, SLE).
- Type IV hypersensitivity
Delayed or cell-mediated reactions are mediated by special immune cells called the T-cell lymphocytes.
The T cells take from a few hours to a few days to mount an allergic response. Examples include contact dermatitises such as poison ivy rashes.
Allergies commonly signify Type I hypersensitivity. This includes allergic rhinitis characterized by runny nose, and eyes and sneezing.
Two major categories are seasonal allergic rhinitis (SAR) and perennial allergic rhinitis (PAR). While SAR is associated with exposure to pollen at certain seasons PAR occurs almost all around the year.
Allergic rhinitis affects an estimated 20-40 million people in the United States.
Other varieties of type 1 reaction are food and drug allergy and allergies due to insect venom.
Insects that may lead to allergies include bees, wasps, yellow jackets, ants, hornets etc.
Allergic asthma is also a type 1 allergic reaction. This occurs when the allergen is inhaled.
Common allergens include pollen, animal dander, fungal spores or moulds, dust mites etc. There is severe wheezing, shortness of breath, cough and thick mucus secretions.