Cutaneous and systemic plasmacytosis (CSP) is an exceedingly rare, chronic, benign, polyclonal, lymphoplasmacytic disorder that is predominantly reported in Asian populations, especially in individuals of Japanese descent. A few cases in White individuals have also been reported.

Initially, this condition was named cutaneous plasmacytosis (CP), but this designation was subsequently changed to CSP after recognition that cutaneous and systemic involvement occur on a spectrum, and widespread, systemic, lymphoplasmacytic infiltration frequently occurs.

CSP has a slight male predominance, and most cases occur between the ages of 30 and 50 years, but pediatric cases of CSP have been recorded. The exact etiopathology is unknown, but suggested pathophysiological considerations include genetic polymorphisms leading to elevations of interleukin 6 (IL-6) and producing dysregulation of mature B-lymphocyte differentiation. Furthermore, isolation of immunoglobulin G4 (IgG4)-positive plasmacytes in lesional skin point toward the possible implication of IgG4 antibodies in the pathogenesis of CSP.

Classically, multiple disseminated, ill-defined, reddish, violaceous, or dark brown, infiltrated papules, nodules, and/or plaques are seen in a symmetric distribution on the back and posterior neck following a "Christmas tree" pattern. Axillary, facial, and limb involvement have also been described. Ulcerative lesions are rarely reported. Pruritus is inconstant, with sporadic or mild-to-moderate intensity pruritus reported in most cases.

Extracutaneous sites of involvement include the lymph nodes (lymphadenopathy is the most common extracutaneous sign), bone marrow, lung (interstitial pneumonia), liver, spleen, and kidneys.

Polyclonal hypergammaglobulinemia is seen in most cases, usually IgG or IgA. IgG levels above 5000 mg/dl have been correlated with systemic involvement.