CD4 and infection: Introduction of viral antigens and adjuvant into the skin activates resident innate cells, including dermal CD14+ dendritic cells, Langerhans cells and mast cells that secrete cytokines, cross present to CD8+ T cells and prime CD4+ T cells to induce switching of naïve B cells into immunoglobulin G (IgG)- and IgA-producing isotypes, with the major immune correlates that consist of IgG and cellular responses deriving from peripheral vaccination or natural infection, whereas higher IgA production is associated with infection or vaccines that act at mucosal sites [17–23].