A 7-yeаr-оld child lives in а cоmmunity withоut а sewage system, and wastewater collects in the streets from toilets and latrines. The child playing in the street becomes infected with the filariform larvae of Strongyloides stercoralis. CD4 cells which encounter the organisms secrete interleukin-4 (IL-4). Which of the following classes of immunoglobulin is most likely to be produced in response to this stimulus?