Dissemination of soluble molecules or antigens via the blood stream is considered to lead to a uniform distribution in the various organs of the body, but organ-specific microarchitecture and vascularization may influence this. Following intravenous injection of alphaCD3epsilon antibody (alphaCD3epsilonAb) we observed clear differences in antibody binding to Fcgamma receptor (FcgammaR)(+) antigen-presenting cells (APCs) or T lymphocytes in different organs. Significant binding of blood-borne alphaCD3epsilonAb was only detected in the spleen and liver and not in the thymus or lymph node. In the spleen, only 10% of dendritic cells/macrophages and 40% of T-cell receptor (TCR)-beta(+) cells were positive for alphaCD3epsilonAb, and, dependent on FcgammaR-mediated cross-linking of alphaCD3epsilonAb, a similar percentage of splenic TCR-beta(+) cells were stimulated and became CD69(+). Stimulation of TCR-beta(+) cells in the liver was at least as efficient as in the spleen, but almost all T cells and all scavenger liver sinusoidal endothelial cells bound alphaCD3epsilonAb. In contrast to CD69 up-regulation, only CD4(+) natural killer T (NKT) cells and CD11a(high) CD8(+) T cells were activated by alphaCD3epsilonAb and expressed interferon (IFN)-gamma. Again, IFN-gamma release from NKT/T cells was at least as efficient in the liver as in the spleen. Taken together, our results support the notion that the combination of extensive hepatic vascularization and very high scavenger activity allows the liver to fulfill its metabolic tasks and to promote stimulation of the large but widely distributed hepatic population of NKT/T cells.