From Pollen to Panic: Demystifying How Allergic Reactions Happen

Joseph R. Anticaglia, MD
Medical Advisory board

Springtime is “allergytime” for millions of Americans. But inhaling substances like pollens is not the only way to provoke an allergic reaction. Breathing in animal dander, dust, mold and mildew can cause an allergic response. The list of allergic “provocateurs” is a long one.

Eating foods such as peanuts, shellfish and eggs, or taking medications like amoxicillin can cause an allergic reaction. Skin contact with certain metals, latex, or poison plants, for instance, poison oak or poison ivy can lead to an allergic response. Injections with medications or insect stings caused by bees and wasps, as well as animal scratches, can trigger an allergic reaction causing mild to life threatening symptoms.

Springtime is ‘allergytime’ for millions of Americans


Typical seasonal allergy symptoms include runny nose, itchy eyes, cough and hives. They’re caused by pollens, a yellowish powder that’s made from grasses trees or weeds. Many allergic reactions are mild, however, an extreme, life-threatening allergic reaction called anaphylaxis is a true emergency. It can cause swelling of the larynx (voice box), difficulty breathing, a drop in a person’s blood pressure and collapse.

What is an Allergic Reaction?

An allergic reaction is an overreaction of the immune system to substances, called allergens, which are normally harmless. Eating, inhaling, touching or injecting allergens can provoke an allergic reaction in people sensitive to such allergens.

How an Allergic Reaction Happens — Three Stages

A) Sensitization Stage — Initial Exposure

In this stage, a person with a genetic predisposition to develop allergic reactions is initially exposed to and becomes sensitized to a specific allergen, for example, tree pollen (which is otherwise harmless to most people). This causes the body to make immunoglobulin E antibodies (IgE) to fight the allergic substance. The IgE antibodies are predominantly found in tissues of the body, such as the mucosa of the nose. IgE cells have memory and can recognize specific allergens; in the above example, tree pollen. There are no symptoms in this stage.

B) Activation Stage — Subsequent Exposure

In this stage, the person experiences a subsequent exposure to tree pollen. The immune system is activated. The IgE antibodies are attached to white blood cells called mast cells and basophils. Immunoglobulin E antibodies in the nose recognize, and capture the tree pollen. In doing so, they trigger a series of reactions, leading to the release of several chemicals, including histamine. Histamine acts on the nose, throat, lungs, eyes, skin, or gastrointestinal tract causing many allergy symptoms.

When histamine, for example, comes in contact with nerve endings in the nose, it can lead to sneezing and itching. It can cause small vessels in the nose to swell causing nasal congestion. Histamine can trigger glands to produce mucus, causing a runny nose. It can constrict muscles in the lungs making it harder to breathe; cause itchy eyes, and throat irritation.

C) Latent Stage — Repeated Exposure

Chronic allergic conditions can weaken the immune system. It can lead to chronic inflammatory conditions, such as chronic rhinitis, allergic asthma, gastrointestinal problems, and lung infections (an allergic reaction that causes inflammation of the lungs — hypersensitivity pneumonitis).

According to the CDC’s National Center for health statistics, “nearly one in three United States adults and more than one in four United States children reported having seasonal allergy, eczema, or food allergies in 2021.” This means that millions of people in the United States, experience various types of allergies each year. Better understanding how allergic reactions occur has led to medications such as antihistamine, and others, which offer relief to allergy suffers.


Anaphylaxis is a life-threatening emergency that has a rapid onset from minutes to hours. It is the most severe form of an allergic (hypersensitivity) reaction.

Mast cells and basophils are a type of white blood cell which are the main source of histamine in the immune system.


  1. Joseph R. Anticaglia, MD; Allergy Season — Allergy Prevention — Allergy Is Bustin’ Out All Over; Doctors Column HC Smart; April 2023
  2. CDC, National Center for Health Statistics; More than a quarter of United States adults and children have at least one allergy; January 26, 2023
  3. Joseph M. Dougherty, et al; Allergy; July 31, 2023, StatPearls
  4. W J Meggs; Mechanisms of allergy and chemical sensitivity; Toxicol Ind Health, Apr-Jun 1999

This article is intended solely as a learning experience. Please consult your physician for diagnostic and treatment options.