Allergic Response

Core Concepts

The two-phase story: sensitization, then reaction

Phase 1 — Sensitization (no symptoms). On first contact with an allergen, the immune system processes it and produces allergen-specific IgE. That IgE locks onto the surface of mast cells (in tissues) and basophils (in blood), where it waits. The person feels nothing; they are now "sensitized."

Phase 2 — Elicitation (symptoms). On later contact, the allergen binds to two adjacent IgE molecules on a mast cell, like a key fitting two locks at once. That cross-linking triggers degranulation: the cell releases histamine and other mediators within minutes. This is why reactions to things like bee stings or injected drugs can appear almost instantly — but only after a prior, symptom-free exposure.

The main players: IgE and mast cells

IgE is the antibody class devoted to defending against parasites, and in allergic people it is produced in response to harmless allergens instead. Mast cells are the warehouses: each one is packed with granules of histamine and other chemicals. The IgE–mast cell partnership is the engine of the immediate allergic response. Note that some substances (such as certain drugs or contrast agents) can trigger mast cells to release histamine without IgE — so a reaction does not always prove a true allergy.

What histamine does

Histamine is the star mediator of the early response, and its effects explain most allergic symptoms:

• Vasodilation — blood vessels widen, causing redness and warmth.
• Increased vascular permeability — vessels leak fluid into tissues, causing swelling (hives, angioedema) and a drop in blood pressure if widespread.
• Smooth muscle contraction — airways narrow (wheezing), and the gut cramps.
• Mucus production and itching — runny nose, watery eyes, scratchy skin.

Hours later, a late-phase response can follow as other immune cells (eosinophils, basophils, lymphocytes) are recruited to the site, causing prolonged congestion and inflammation — why allergic nasal symptoms can outlast the initial exposure.

The four hypersensitivity types: where allergy fits

Immunologists classify hypersensitivity into four types. Allergy as usually meant in this chapter is type I, but the other types explain related disorders:

Local vs. systemic reactions

An allergic response can stay local — a patch of hives, a stuffy nose, an itchy rash — or it can become systemic, involving multiple organ systems at once. Systemic involvement (skin plus respiratory plus cardiovascular or gastrointestinal signs) is the warning pattern of anaphylaxis, the most dangerous end of the spectrum, covered later in this chapter.

The nursing role: history, documentation, education

• Take a structured allergy history: what substance, what reaction, how soon after exposure, how severe, what treatment was needed, and whether the person has been re-exposed since. Write down the person's own words and the timing.
• Document clearly per facility policy, and make sure allergies are visible in the record and communicated at handoff.
• Verify before you assume. "Reported allergies" are self-reported; when records are vague or conflict, follow the facility's process for clarifying them with the patient or a provider — do not silently ignore or silently accept a label.
• Educate: teach the person the name of their allergen, how to read labels, how to recognize early symptoms, when to seek help, and how to communicate their allergy to every care team (including EMS and dentists).
• Scope note: allergy testing and immunotherapy are provider-directed; the nurse's role is to prepare, monitor, and report — protocols vary by institution and jurisdiction.

Common Confusions