What is Mast Cell Activation Syndrome? Etiology and Pathology

Mast Cell Activation Syndrome (MCAS) is a mast cell disorder. There is primary, secondary and idiopathic.

It is distinct from Systemic Mastocytosis because there is no excess accumulation of mast cells, and distinct from primary mast cell disorders because it is not IgE mediated.

It is characterised by inappropriate or excessive mast cell activation leading to a wide range of inflammatory and allergy-like symptoms. 

MCAS affects both children and adults and may present at any age. Symptoms are often long-standing but under-recognised, with many patients reporting mild or intermittent symptoms from childhood before a sudden escalation in severity.

In some cases, a clear precipitating event precedes deterioration, such as:

• Viral or bacterial infection
• Surgery or medical procedures
• Physical or psychological stress

Others are unwell from birth or early childhood with no clear precipitating event.

The condition is frequently characterised by unpredictable, relapsing–remitting symptoms involving multiple organ systems.

Mast cells release hundreds of biologically active mediators when triggered; the full extent of their effects is an evolving area of research.

Preformed mediators are released immediately, while newly synthesised mediators are released over minutes to hours after exposure to a trigger.