BACKGROUND Aberrant expression of CD23 (low affinity IgE receptor) on cells of the monocyte/macrophage series in peripheral blood and lesional skin of patients with atopic eczema has been demonstrated. It is not known whether this abnormality results from a fundamental systemic problem of the monocytes of these patients or reflects local changes to cell populations within the skin tissues. OBJECTIVE This study was designed to determine whether this aberrant expression was caused by local cutaneous influences on mature cells or fundamental changes in monocyte differentiation. The possible relationship between these aberrations and clinical severity was also investigated by repeating these immunopathological studies after a course of efficacious treatment with Chinese herbal therapy (CHT). METHODS Peripheral blood mononuclear cells were obtained from patients with atopic eczema before, and after 8 weeks of treatment. Efficacy of CHT was quantified on clinical grounds. Monocytes were isolated by adherence to plastic and cultured for up to 7 days. Samples were harvested at 2, 5 and 7 days of culture and cytospins prepared. Immunocytochemical staining to identify phenotypic subsets was performed on the monocytes at time 0 and on maturing cells from culture. This immunocytology was quantified using computerized image analysis equipment to determine the emergence of macrophage subsets and their level of CD23 expression. Biopsies were taken from lesional skin before and after treatment and immunohistology was performed on cryostat sections to determine the number of antigen presenting cells expressing CD23 as well as the level of expression of these molecules. RESULTS The results showed that increased numbers of monocytes from patients with atopic eczema express CD23 at day 0 and that cultured monocytes from these patients differentiate faster during the 7 day culture period as compared to normal controls. Efficacious treatment did not affect the number of peripheral blood monocytes expressing CD23. However, treatment did lead to a significant decrease in the number of CD23+ mature macrophages in the skin as well as a reduction in the level of expression of this moiety. These results demonstrate that changes in clinical severity are more closely related to the expression of CD23 on mature antigen presenting cells in lesional skin rather than to differentiating peripheral blood monocyte CD23 expression. CONCLUSIONS These results suggests that local factors within lesional skin govern the accumulation and the expression of CD23 on mature macrophages and that these factors may be more relevant to the pathogenesis of the disease than aberrations in CD23 expression that may occur systemically.