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 predominant cutaneous involvement. Investigator's Global Assessment (IGA) score of 3 or 4 at screening and randomization. * Patients with nonsegmental vitiligo (NSV), documented clinical diagnosis of NSV for at least 3 months. Facial-Vitiligo Area Scoring Index (F-VASI) ≥ 0.5 and Total body-VASI ≥ 3 at screening and randomization. Exclusion Criteria: * Participants who have a history of severe allergic reactions or hypersensitivity to the active ingredient and excipients of the study drug or drugs of the same class. * Participants who are unable to comply with the requirements of the protocol, unless the written approval of the medical monitor has been obtained before informed consent. * Participants with any malignancy ≤ 5 years before randomization, except for any locally recurring cancer that has been treated curatively * Participants who were administered a live vaccine ≤ 28 days before randomization. * Female participants who are pregnant or are breastfeeding. * History of Tuberculosis or active, latent, or inadequately treated infection * A known history of a primary immunodeficiency or an underlying condition such as HIV infection or splenectomy that predispose the participant to infections. * Known infection with hepatitis B virus \[presence of hepatitis B surface antigen (HBsAg) or hepatitis B core antibody( HBcAb)\], or presence of hepatitis C virus antibody (HCV Ab) * Have a history of any lymphoproliferative disorder (such as Epstein Barr Virus-related lymphoproliferative disorder, as reported in some participants on other immunosuppressive drugs), history of lymphoma, leukemia, myeloproliferative disorders, multiple myeloma, or signs and symptoms suggestive of current lymphatic disease. * Have an active infection or a history of infection within 6 months before the first dose of study drug that required hospitalization, or parenteral antimicrobial therapy, or have a history of opportunistic infection or a chronic or recurrent infectious disease that, in the opinion of the investigator, makes them unsuitable for this study. * Have or have had symptomatic herpes zoster or herpes simplex within 12 weeks, more than 1 episode of local herpes zoster, or a history (single episode) of disseminated zoster. * Acute disease state (e.g., asthma attack, nausea, vomiting, fever, or diarrhea) within 7 days prior to the first dose of study drug. Part B(MAD) specific exclusion criteria: * Previous use of any Janus Kinase (JAK) inhibitor for any disease indication at any time. * Treatment with systemic immunomodulatory medications within 4 weeks or 5 half-lives (if known), whichever is longer prior to randomization, including immunosuppressants (e.g., methotrexate, cyclosporine, azathioprine); corticosteroids administered orally, intravenously, or intramuscularly; chloroquine derivatives; and oral phosphodiesterase-4 (PDE4) inhibitors (e.g., apremilast). * Phototherapy (UV or laser therapy) or cryotherapy within 4 weeks prior to randomization. * For AA patients, AA that affects more than scalp hair, e.g., eyebrow, eyelash, body hair. * For CLP patients, Patients whose LP is a predominantly bullous variant. * For NSV patients, Participants who have no pigmented hair within any of the vitiligo areas on the face. Note: Other protocol defined Inclusion/Exclusion criteria may apply.