A Study of LAD603 in Adults With Alopecia Areata

Inclusion Criteria: * Participant is a male or female between 18 and 65 years old at the time of signing the informed consent. * Severe to very severe AA criteria: 1. Greater than or equal to (\>=) 50% hair loss of the scalp as measured by SALT without evidence of terminal hair regrowth within 6 months at both the Screening and Baseline (Day 1) visits. 2. Current episode of hair loss lasting at least 6 months and not exceeding 7 years. * Participants who are WOCBP or male must agree to the requirements for the avoidance of pregnancy and exposure of the participant's partner to LAD603 during the study, Exclusion Criteria: AA, Skin Specific, and Other Inflammatory Diseases * Participants have other types of alopecia (including but not limited to traction, scarring alopecia). * Participants with "diffuse" type AA. * Currently has an active form of other inflammatory skin disease(s) or evidence of skin condition (e.g., psoriasis, seborrheic dermatitis, lupus) at the time of the Screening or Baseline (Day 1) visits that is expected to interfere with the assessment of alopecia areata severity. Other Medical Conditions * Participant has previous severe adverse reaction to subcutaneously administered medication. * Participant has any of the following liver safety laboratory results at Screening 1. Alanine aminotransferase (ALT) or aspartate aminotransferase (AST) greater than or equal to (\>=) 2.5 × upper limit of normal (ULN) 2. Total bilirubin (TBL) \>=1.5 × ULN (TBL \>=3 × ULN in participants with Gilbert's syndrome) 3. Alkaline phosphatase (ALP) \>=1.5 × ULN Prior/Concomitant Therapy * Intention to use any concomitant medication that is not permitted by this protocol or failure to undergo the required washout period for a particular prohibited medication.