Moreover, treatment with neutralising antibodies may also increase cytokine levels in the circulation; for example, mepolizumab-treated patients had higher circulating levels of IL-5 with the majority of it bound in IL-5–anti-IL-5 complexes [90], and treatment of asthma patients with anti-IL-13 antibodies directly increased serum levels of IL-13 [91]. This evidence concerns the gene IL13 and asthma.