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) T_H2 lymphocytes produce a cytokine (type of chemical mediator or signalling molecule) called Interleukin-4 (IL-4)	B-cells (produce antibodies) Interact with T_H2 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.

PART 2: DEGRANULATION 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.

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 T_H2 lymphocytes.
If a late-phase response occurs, it typically develops between 2 to 24 hours after exposure to the allergen.