Brent Cornell

    
An allergen is an environmental substance that triggers an immune response despite not being intrinsically harmful

• This immune response tends to be localised to the region of exposure (e.g. airways and throat) as an allergic reaction
• A severe systemic allergic reaction is called anaphylaxis and can be fatal if left untreated 

An allergic reaction requires a pre-sensitised immune state (i.e. prior exposure to the allergen)

• When a specific B cell first encounters the allergen, it differentiates into plasma cells and makes large quantities of antibody (IgE)
• The IgE antibodies attach to mast cells, effectively ‘priming’ them towards the allergen
• Upon re-exposure to the allergen, the IgE-primed mast cells release large amounts of histamine which causes inflammation

Stages of an Allergic Reaction

    
The release of histamine from IgE-primed mast cells causes an inflammatory response that results in allergic symptoms

• Inflammation improves leukocytes mobility to infected regions by triggering vasodilation and increasing capillary permeability

Vasodilation is the widening of blood vessels to improve the circulation of blood to targeted regions

• Vasodilation causes redness (as vessel expansion moves blood closer to the skin) and heat (which is transported in blood)

Capillary permeability describes the capacity for leukocytes to leave the bloodstream and migrate into the body tissue 

• Increased permeability leads to swelling (more fluid leaks from the blood) and pain (swelling causes compression of nerves)

Redness, heat, swelling and localised pain are all typical symptoms of an allergic response

Inflammatory Response