Allergic to Dogs and Allergies in Dogs Resource

Allergy Response – Sequence of Events of an Allergy Reaction

There are 2 possible phases of a typical allergy response (i.e. Type 1 Hypersensitivity) mounted by the immune system of the host in response to an allergen, but the later one (late-phase response) may not always develop.

1. Acute Response i.e. early-phase

PART 1: Production of SENSITIZED cells
Occurs on exposure to allergen for the FIRST time

The allergen encounters an APC cell
(professional Antigen-Presenting Cell)
which causes the following response:

White blood cells (WBC)

Lymphocytes (type of WBC)

T-cells (involved in immune response)
TH2 lymphocytes produce a cytokine
(type of chemical mediator or signalling molecule) called Interleukin-4 (IL-4)
B-cells (produce antibodies)
Interact with TH2 lymphocytes and IL-4 to produce IgE
IgE is secreted into blood and coats specific immune cells (mast cells and basophils) by binding to IgE-receptors (FcεRI) on their surface. This produces cells that are now sensitized to the allergen.

Occurs on SUBSEQUENT exposure to allergen

  • The allergen and sensitized cells bind, which activates structures (granules) to release inflammatory chemical mediators. This process is known as degranulation.
  • The released inflammatory chemical mediators will then carry out their modus operandi resulting in the symptoms of an allergic response. The nature and severity of the symptoms will depend on a number of variables including the individual, the allergen and its mode of introduction.
  • In a person sensitized to a given allergen, the acute response phase occurs within seconds or minutes of subsequent exposure to the allergen.

Allergy Response  Sequence of Events of an Allergy Reaction


 2. Late-Phase Response

  • As the inflammatory chemical mediators of the acute response decline, there is an influx of various white blood cells (eosinophils, lymphocytes, macrophages and neutrophils) to the affected areas.
  • Mast cell cytokines (type of signalling molecule or chemical mediator) and inflammatory chemical mediators released from eosinophils appear to play a notable role, as does the recruitment of TH2 lymphocytes.
  • If a late-phase response occurs, it typically develops between 2 to 24 hours after exposure to the allergen.
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