A Study to Evaluate the Safety of BG-A3004 in Healthy Participants and Patients With Immune-Mediated Skin Diseases

Inclusion Criteria * Evidence of a personally signed and dated informed consent document indicating that the participant has been informed of all pertinent aspects of the study. * Female participants of childbearing potential must be willing to use a highly effective method of birth control and refrain from egg donation for the duration of the study and for 180 days after the last dose of study drug. They must also have a negative urine pregnancy test at baseline before first dose of study drug. * Nonsterile male participants must be willing to use a highly effective method of birth control and refrain from sperm donation for the duration of the study and for 180 days after the last dose of study drug. Part A (SAD) specific Inclusion criteria * Healthy participants as determined by medical evaluation, including medical history, physical examination, laboratory tests, and cardiac monitoring. * must be 18 to 55 years of age, inclusive, at the time of signing the informed consent. * Body mass index (BMI) of 19 to 28 kg/m\^2 Part B (MAD) specific inclusion criteria: * Must be 18 to 60 years of age, inclusive, at the time of signing the informed consent. * Body mass index (BMI) of 18 to 32 kg/m\^2 * Patients with alopecia areata (AA), clinical diagnosis of AA with no other etiology of hair loss. Severity of Alopecia Tool (SALT) ≥ 25 and \< 95 at screening and randomization. * Patients with cutaneous lichen planus (CLP), clinical and histological features of LP with predominan…