Molecular model of a human interleukin-4 receptor subunit alpha (red, blue) complexed with IL-4R nanobody HuNb103 (pale yellow-green, cyan). Interleukins are a group of cytokines (secreted proteins and signal molecules) that are expressed and secreted by white blood cells (leukocytes) as well as some other body cells. Interleukin 4 is produced by CD4+ T cells specialized in providing help to B cells to proliferate and to undergo class switch recombination and somatic hypermutation.