Background Different types of circulating dendritic cells have been described. Dendritic cells influence differentiation of naive T lymphocytes into T helper type 1 (Th1) and Th2 effector cells.

Objective The purpose of this study was to evaluate the number of circulating DC subtypes in peripheral blood of allergic and healthy children and in cord blood of neonates from allergic and non‐allergic parents.

Methods Circulating dendritic cells were flow cytometrically identified in whole blood samples as lineage (CD3, CD14, CD16, CD19, CD20, CD56) negative, CD34 negative and HLA‐DR‐positive cells. According to the expression of CD123 and CD11c, different DC subtypes were identified.

Results Apart from DC1 (CD11c⁺ CD123^dim+) and DC2 (CD11c^– CD123^high+), a third DC population was described with less differentiated phenotypic characteristics, namely CD11c^– CD123^dim+, and therefore defined here as less differentiated DC (ldDC). These ldDC represented the major DC population in cord blood and showed an age‐depended decrease. The highest level of ldDC was detected in children with atopic dermatitis, whereas asthmatic children showed the lowest ldDC counts. Furthermore, high‐dose inhaled corticosteroid treatment in asthmatic children was related to a decreased ldDC number. The number of circulating DC2 was significantly lower in allergic children, especially in asthmatics, compared to healthy children. In cord blood, no differences in DC subtypes were detectable between neonates at low and high risk for allergic disorders.

Conclusion These results indicate that, apart from DC1 and DC2, a third population of dendritic cells, identified as CD11c^– CD123^dim+ cells and defined as less differentiated DC, must be considered in the evaluation of circulating DC. Furthermore, DC2 counts were decreased in allergic children, especially in asthmatics, which might be the consequence of an increased recruitment to the target organs.