**3. Histopathology of vasculitis**

The histopathological characteristics of KD are as follows: (1) major muscular arteries branching from the aorta, including the coronary arteries, are predominantly injured; (2) the damaged arteries are extra-arterial, not arteries within organs; (3) acute vasculitis occurs synchronously throughout the body; and (4) vasculitis is a proliferative inflammation consisting of an abnormal accumulation of monocytes/ macrophages.

KD is characterized by inflammation of the coronary artery in the acute phase, which usually lasts for approximately 6 weeks. The earliest histological changes in coronary arteritis are seen on sixth to eighth day of illness, starting with the infiltration of inflammatory cells in the tunica adventitia and tunica intima. Inflammatory cells infiltrate the tunica media, leading to inflammation of all layers of the vessel wall by the tenth day of illness. Subsequently, the artery begins to dilate owing to significant damage to the internal elastic lamina or tunica media. Inflammatory cell infiltration continues for approximately 2 weeks and then gradually fades. If the vessel wall undergoes a certain degree of damage, even after vasculitis subsides, inflammatory scarring of the coronary artery remains for a long time. In particular, in patients with coronary aneurysms, various findings, such as stenotic lesions or extensive calcification of the aneurysm wall, are observed [11].

In addition to the coronary arteries, other systemic blood vessels are injured by vasculitis [12]. Whole-body examination for KD to evaluate systemic vasculitis shows vascular damage at various sites, especially in the subclavian, brachial, axillary, and iliac arteries. Many case reports have revealed that systemic arterial aneurysms are almost always associated with giant coronary arterial aneurysms, and a detailed evaluation should be considered in these patients.
