A routine complete blood count performed on a 22-year-old medical student reveals an abnormality in the differential leukocyte count. She has been complaining of frequent sneezing and "watery" eyes during the past several weeks and repos that she frequently had such episodes in the spring and summer. Which of the following cell types is most likely to be increased in the last phase reaction?

This type of reaction is primarily mediated by the release of histamine from tissue mast cells In response to antigen and other stimuli, including cytokines produced at the local site, the T cells differentiate into Th2 cells, which secrete IL-4, IL-5, and IL-13. In the late-phase reaction, leukocytes are recruited that amplify and sustain the inflammatory response without additional exposure to the triggering antigen. Eosinophils are often an abundant leukocyte population in these reactions. IL-5 is involved in the development and activation of eosinophils, which are impoant effectors of type I hypersensitivity. The associated cellular infiltrate and peripheral blood findings represent mobilization and increased numbers of eosinophils. The symptoms repoed are those of seasonal rhinitis, better known as "hay fever," a manifestation of type I hypersensitivity.