Serotonin initiation of delayed-type hypersensitivity: mediation by a primitive Thy-1+ antigen-specific clone or by specific monoclonal IgE antibody.

Elicitation of delayed-type hypersensitivity (DTH) responses is due to the required sequential action of two different antigen (Ag)-specific Thy-1+ cells: early acting, DTH-initiating cells and locally recruited CD4+, alpha beta-TCR+, DTH effector T cells. DTH-initiating cells have an unusual phenotype for Ag-specific cells (Thy-1+, CD5+, CD4-, CD8-, CD3-, sIg-, B220+ (CD45RA+), Mac 1+, IL-2R- and IL-3R+) and act by producing Ag-specific non-IgE factors that sensitize mast cells for release of the vasoactive amine serotonin (5HT) at the local site of elicitation of DTH by challenge. Another mechanism of DTH initiation involves Ag-specific IgE antibodies that also can sensitize mast cells for local serotonin release. Serotonin initiates DTH by activating the local endothelial cells to allow recruitment of DTH effector T cells, and also by activating 5HT-2 receptors on these recruited T cells.