A Study of LY3454738 in Healthy Participants and Participants With Atopic Dermatitis

Inclusion Criteria: * All: Must have a body mass index of 18.0 to 32.0 kilograms per square meter (kg/m²), inclusive, for Parts A and B, and 18.0 to 45.0 kg/m², inclusive, for Part C and a minimum body weight of 50 kilograms * For PART A and B only, regarding the inclusion of healthy Japanese participants: Minimum age is 20 years and the participant's biological parents and all of the participant's biological grandparents must be of exclusive Japanese descent and must have been born in Japan * AD: * Must have a diagnosis of AD for at least 12 months with either poor response to topical treatments or inability to use topical treatments * Must agree to use moisturizer daily throughout the treatment period Exclusion Criteria: * All: must not have previously completed a clinical trial with a molecule targeting interleukin-33 (IL-33) or its receptor * AD: * Must not have received certain topical medications for AD within 2 weeks prior to randomization * Must not have received certain oral medications for AD or received phototherapy within 4 weeks prior to randomization * Must not have received any antibody-based biologic agents (marketed or investigational) within 5 half lives (t1/2) of the drug prior to randomization * Must not have received intravenous, intramuscular, or intra-articular steroids in the past 6 weeks prior to randomization