Introduction

The primary (idiopathic) systemic vasculitides constitute a heterogeneous group of diseases characterized by inflammation of blood vessels. These are typically multisystem disorders although isolated (confined to a single organ) vasculitis is well recognized (Carrington & Liebow, 1966; Borrie, 1972; Orbo & Bostad, 1989; Warfield et al., 1994). The aetiology has yet to be defined but is likely to be multifactorial involving an initiating trigger, possibly infection (Davies et al., 1982; Pinching et al., 1984; DeRemee, McDonald & Weiland, 1985; Savage et al., 1991; Barrett et al., 1993; Stegeman et al., 1994), in genetically susceptible individuals (Katz et al., 1979; Elkon et al., 1983; Hay et al., 1991; Spencer et al., 1992). Further, because there is considerable clinical and histological overlap within the group, this has made advances in the classification difficult and has also led to confusion in terminology. The most clinically useful classification remains that based on the size of blood vessels involved (Adu, Luqmani & Bacon, 1993; Scott, 1993). The problem of nomenclature has been specifically addressed by the Chapel Hill international consensus (Jennette et al., 1994).

In this chapter we will focus on the small vessel, necrotizing systemic vasculitides, Wegener's granulomatosis and microscopic polyarteritis. It is clear from histological studies in this group that the vascular endothelium is a likely target for initial injury, preceding the development of vessel wall or extravascular inflammation. We shall discuss the proposed mechanisms of this endothelial injury and inflammation in vasculitis.