As the most distal branches of the meridian system,tertiary collaterals(sunluo)are morphologically similar to microvessels in western medicine,while Henoch-Sch?nlein purpura(HSP)is a manifestation of systemic vascular inflammation.The discussion in Huang Di Nei Jing(The Yellow Emperor's Inner Classic)regarding tertiary collaterals"draining pathogenic factors and regulating ying-wei"provides valuable theoretical guidance for the traditional Chinese medicine(TCM)treatment of HSP.This article thoroughly analyzes the physiological functions of tertiary collaterals and proposes that the formation of HSP is due to the invasion of pathogenic factors into the tertiary collaterals,leading to the imbalance of ying-wei,heat stasis obstructing the vessels,and blood extravasation,resulting in symptoms such as skin purpura,gastrointestinal bleeding,and joint pain.These symptoms further interfere with the convergence and biochemical processes of ying-wei,causing an imbalance of body fluids and blood.The overflow of body fluids from the tertiary collaterals,unable to return,leads to fluid stagnation.The accumulation of blood stasis and dampness in the kidney collaterals may cause kidney damage,manifesting as hematuria and proteinuria.The pathogenesis at different stages of the disease can be summarized as follows:in the early stage,pathogenic factors invade the tertiary collaterals,causing wei stagnation and ying obstruction;in the middle stage,fire toxin scorches the tertiary collaterals,affecting ying-wei,damaging yang and yin;or damp pathogens invade the lower regions through the tertiary collaterals,affecting the yin organs;in the late stage,deficiency of vital qi and collateral stasis lead to kidney dysfunction.Based on these pathological mechanisms,the treatment principle should focus on"harmonizing pathogenic wind and blood circulation,and unblocking the tertiary collaterals,"aiming to alleviate purpura symptoms,shorten the disease course,promote recovery,and achieve comprehensive prevention and treatment effects.