Inflammatory mediators released from activated mast cells and basophils during hypersensitivity reactions have direct pathological effects on the myocardium and coronary vasculature. It was traditionally thought that cardiovascular signs and symptoms in anaphylaxis are largely due to peripheral vasodilation and increased vascular permeability. However, there is extensive evidence of primary cardiac involvement during hypersensitivity reactions, most notably coronary vasoconstriction as well as atherosclerotic plaque erosion and rupture, leading to angina pectoris and acute coronary syndromes. Furthermore, mast cells are well established as effector cells in atherosclerosis, through their effects on atherosclerotic plaque progression and destabilisation. It was noted over 30 years ago that cardiac patients have a markedly higher concentration of biologic amines (especially histamine) in their coronary vasculature, and, additionally, are hyper-reactive to the effects thereof. This is borne out by the disproportionate mortality rate of those with cardiac disease that suffer a hypersensitivity reaction. Kounis syndrome refers to angina pectoris or an acute coronary syndrome secondary to a hypersensitivity reaction, with the subtypes dependent on the underlying state of the coronaries and presence of a drug-eluting stent or not. This review will focus mainly on the aetiology, pathophysiology, diagnoses and treatment of this important syndrome.