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VASCULITIS
An attempt to define the nature of vasculitis is compromised by the broad nature of this category. A basic
requirement might be that the disease should be generalized, or disseminated, but this approach is easily defeated by examples that are anatomically limited. An approach that admits only examples of immune-complex
disease can be countered by examples of viral diseases in which the process is said to be cell-mediated. Attempts to rigidly define immune processes, as being either humoral or cell-mediated, will be foiled by a
realization that the defense processes of the body normally do not function solely in one realm or the other; cell-mediated reactions usually are part, and parcel, of immune complex disease.
B-lymphocytes obviously are involved in immune complex disease (S8C9VA1-2-4). Serum sickness is often cited as the model for immune complex disease (S8C9VA1-6 & 7). The Arthus reaction also is a model (S8C10VA2-1 & 2).
An immunological classification of the vasculitides places emphasis on the character of the antibodies, and of
the immune complexes.
In the morphologic classification of the vasculitides, groups can be defined on the basis of the distribution of
the vascular changes in the organ systems and the character of the inflammatory infiltrates (S8C9VA1-2, & S8C9VA1-5). In addition, lesions can be grouped according to the size of vessels involved by the inflammatory
process. Distinctions can be made on the character of the inflammatory infiltrates; some examples are neutrophilic, some are granulomatous, and some are rich in eosinophils. Some are thrombotic with fibrinoid
degeneration of the walls of vessels. Some are relatively fixed and chronic (S8C13VA4-3). In the category of the necrotizing
angiitides, components of the clotting system may be altered in regard to their concentration in the blood; these variations are cited as being of some value in establishing a diagnosis of vasculitis (S8C10VA2-4, & S8C13VA4-1 & 2).
Morphologists have adopted a limited approach that is more preached than practiced. The most common examples of
palpable purpura are characterized by neutrophilic infiltrates and leukocytoclasia (S8C9aVA1a-1) but, in
practice, the infiltrates, more often than not, are more uniformly neutrophilic in the interstitium of the reticular dermis (S8C10VA2-3). The perivascular spaces, more often than not, show lymphohistiocytic infiltrates; necrosis of the
walls of blood vessels is not a regular feature. Often fibrin deposits are more prominent in the adventitia of vessels than in the actual walls of vessels. Distinctions are made between leukocytoclastic
angiitis and neutrophilic interstitial dermatitides (i.e., Sweet syndrome, etc.), but the distinctions most often are reflections of the severity of the interstitial infiltrates and the clinical presentation; the
interstitial neutrophilic dermatitides are characterized by infiltrates that are more extensive, more diffuse, and more destructive of dermal connective tissue. In spite of all these inconsistencies, the notion that
necrosis of the walls of vessels is required for a diagnosis of leukocytoclastic angiitis is strongly promoted in open discussions and in the literature. The perivenular lymphocytic infiltrates have legitimacy as
examples of vasculitis (they may even be associated with morphologic evidence of leukocytoclasia), but most observers reserve the designation, lymphocytic vasculitis, for those examples in which a vessel focally
shows areas of fibrinoid necrosis.
In the category of the necrotizing, thrombotic angiitides, necrosis of the walls of vessels may be a prominent
feature in the absence of cellular infiltrates (S8C11P1-1). The Shwartzman reaction has been cited as a model for
the study of thrombotic disorders (S8C14VA5-5-7). In contrast, a cellular infiltrate usually is characteristic of the
vasculitis that is associated with immune complexes (S8C11P1-2). Necrosis, often plate-like in the epidermis (S8C11P1-3) and edema of the papillary dermis (S8C11P1-4-6) are common features of the reaction to a vasculitis. With few exceptions, in the histologic definition
of angiitis reference is made to fibrinoid necrosis in the immediate vicinity of the wall of the affected vessels. If the reacting cell is the neutrophil, then the infiltrates generally extend into the dermis among
collagen bundles of the reticular dermis (S8C11bP1b-1-4). Some of the inflammatory cells disintegrate; they leave behind
fragments of nuclei (some of the nuclear fragments may be associated with remnants of cytoplasm, in which case, the patterns have the qualities of an apoptotic process). The term, leukocytoclasia, gives recognition
to the process of nuclear fragmentation. The intensity of the inflammatory response in leukocytoclastic angiitis is extremely variable; for some examples, the patterns are subtle (S8C11aP1a-1 & 2); they might easily be dismissed as those of a dermatitis, rather than a vasculitis. The cellular
response also is variable; perhaps, some of the variations are related to alterations which occur in inflammatory infiltrates as a measure of age of the selected lesion. The patterns in a young lesion may differ
significantly from those of an old lesion. For some examples, the character of the immune complex, particularly size and solubility, may influence the character of the inflammatory response (S8C11aP1a-3-7 & S8C11P1-8).
In the skin, a vasculitis usually affects small vessels; most often, small vessels in the upper portion of the
dermis are involved. The extent of involvement of the vascular plexus of the skin is not a measure of the likelihood of systemic or disseminated disease; although, the vascular plexus of the skin often is involved
both superficially and deep in disseminated disease. Systemic disease can be characterized as either small vessel disease (small vessel angiitis of Zeek) (microscopic vasculitis?) (S8C11P1-7, & S8C13VA4-4),
or medium and large vessel disease (disease of muscular vessels -classic polyarteritis nodosa). The nature of the immune complex has some correlations with the nature of the clinical disease (S8C12VA3-1-7). Some vasculitides are associated with hypocomplementemia (S8C13VA4-1 & 2). Anatomic distribution of lesions in the category of systemic vasculitides has clinical relevance (S8C14VA5-1-4).
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