Allergens can enter the body through various routes like inhalation, ingestion, injection, and external skin contact.

They react with antibodies in a susceptible person, causing the release of histamine (a chemical mediator in allergic reactions) and other chemical substances that cause various symptoms.

This creates a chain response known as the allergic response.

Causes

Risk factors for allergy can be placed in two general categories, namely host and environmental factors.

Host factors include heredity, sex, race, and age, with heredity being by far the most significant.

Seasonal allergies

Seasonal allergies occur during certain seasons of the year

Perennial allergies

Perennial allergies occur throughout the year

Signs & symptoms

• Many allergens such as dust or pollen are airborne particles
• In these cases, symptoms arise in areas in contact with air, such as eyes, nose and lungs
• For instance, allergic rhinitis, also known as hay fever, causes irritation of the nose, sneezing, and itching and redness of the eyes
  

Diagnosis

• Once a diagnosis of asthma, rhinitis, anaphylaxis, or other allergic disease has been made, there are several methods for discovering the causative agent of that allergy.
• Nasal smear testing- In this a sample of nasal secretions is collected and analyzed in a laboratory.
• Eosinophils (a type of cell present in the body) are produced during an allergic reaction, and these are observed under a microscope in the nasal specimens obtained.

Treatment

• Drugs called antihistamines are used to treat allergies
• Their most common side effect is drowsiness
• Some antihistamines are non-sedating, and do not cause drowsiness

Prevention

The best way to avoid symptoms of allergy is prevention.

This means avoiding exposure to agents that trigger the allergic response. Being aware of common allergens like pollen, dust, fur, drugs, specific foods etc, and avoiding them is absolutely essential in preventing an allergic reaction.