1. Male or female participants who are at least 6 years old and under 18 years old, who can provide assent (if appropriate), and for whom signed informed consent can be provided by parent or legal guardian prior to participation in any study assessments or procedures

• Participant is able to adhere to the study visit schedule and other protocol requirements.

1. Females of childbearing potential (FCBP) must have a negative pregnancy test at Screening and Baseline. While on investigational product and for at least 28 days after taking the last dose of investigational product (IP), FCBP who engage in activity in which conception is possible must use one of the approved contraceptive options described below:

• Option 1: Any one of the following highly effective methods: hormonal contraception (oral, injection, implant, transdermal patch, vaginal ring); intrauterine device (IUD); tubal ligation; or partner's vasectomy; OR
• Option 2: Male or female condom (latex condom or non-latex condom NOT made out of natural [animal] membrane [for example, polyurethane]);

1. Participant has a history of at least 6 months of moderate to severe AA (greater than or equal to 50% scalp involvement) as measured using the SALT score.
2. Participant has a screening IgE greater than or equal to 200 and/or personal and/or familial history of atopy (including asthma, atopic dermatitis, allergic rhinitis, food allergy, or eosinophilic esophagitis)
3. Participant is judged to be in otherwise good overall health following a detailed medical and medication history, physical examination, and laboratory testing.

1. Inability or unwillingness of a participant to give written informed consent or comply with study protocol
2. Participant is pregnant or breastfeeding.
3. Participant's cause of hair loss is indeterminable and/or they have concomitant causes of alopecia, such traction, cicatricial, pregnancy-related, drug-induced, telogen effluvium, or advanced androgenetic alopecia (i.e. Ludwig Type III or Norwood-Hamilton Stage greater than or equal to V).
4. Participant has a history of AA with no evidence of hair regrowth for greater than or equal to 7 years since their last episode of hair loss.
5. Severe, uncontrolled asthma (having 2 or exacerbations in the last 12 months that require systemic steroids and/or hospitalization) or a history of life-threatening asthma exacerbations while on appropriate anti-asthmatic medications.
6. Participant has an active bacterial, viral, or helminth parasitic infections; OR a history of ongoing, recurrent severe infections requiring systemic antibiotics
7. Participant with a known or suspected underlying immunodeficiency or immune-compromised state as determined by the investigator.
8. Participant has a concurrent or recent history of severe, progressive, or uncontrolled renal, hepatic, hematological, intestinal, metabolic, endocrine, pulmonary, cardiovascular, or neurological disease.
9. Known active hepatitis B, hepatitis C, human immunodeficiency virus (HIV), or positive HIV serology at the time of screening for subjects determined by the investigators to be at high-risk for this disease.
10. Participant has a suspected or active lymphoproliferative disorder or malignancy; OR a history of malignancy within 5 years before the Baseline assessment, except for completely treated in situ non-melanoma skin and cervical cancers without evidence of metastasis.
11. Participant has received a live attenuated vaccine less than or equal to 28 days prior to study randomization.

Note: Other protocol defined Inclusion/Exclusion criteria may apply.