A Study to Investigate the Safety and Pharmacological Effect of a Single Intravenous Infusion of Belantamab in Male and Female Participants Aged 18 to 75 With Autoimmune Disease

Inclusion Criteria:

• Body mass index (BMI) from 18 to 40 kilograms per square meter (kg/m^2) (BMI =weight/height^2), inclusive, and body weight of >=40 kilogram (kg)

• IgM >= lower limit of normal (LLN) (40 milligram per deciliter [mg/dL]) at initialscreening visit (ISV)

Participants with systemic lupus erythematosus (SLE) must also meet the following inclusion criteria:

• Documented clinical diagnosis of SLE according to the European League AgainstRheumatism (EULAR) or American College of Rheumatology (ACR) Classification Criteria

• Positive anti-double stranded deoxyribonucleic acid (anti-dsDNA) autoantibody and/orpositive antinuclear antibody (ANA) test results, using central lab test, at ISV.

• SLE Disease Activity Index 2000 (SLEDAI-2K) total score of >=6 at ISV.

• Failure to adequately respond to at least two immunosuppressive therapies.

Participants with Rheumatoid Arthritis (RA) must also meet the following inclusion Criteria:

• Meets ACR/EULAR 2010 RA Classification Criteria with a duration of RA disease of >=6months at time of ISV

• Positive rheumatoid factor (RF) and/or anti-cyclic citrullinated peptide (anti-CCP)test results, using central lab test, at ISV.

• Have moderate to severe active disease as defined by >=3/68 Tender and >=3/66Swollen joint count at ISV and Baseline.

• Failure to adequately respond to at least two immunosuppressive therapies.

Participants with antiphospholipid syndrome (APS) must also meet the following inclusion criteria:

• Documented diagnosis of APS meeting the 2023 ACR/EULAR APS classification criteria

• Positive lupus anticoagulant test or moderate to high titers of positive IgG/IgManticardiolipin (aCL) or moderate to high titers of IgG/IgM anti-beta2-glycoproteinI antibody using central lab test, at ISV

• Clinical features attributable to antiphospholipid antibodies that are resistant towarfarin and/or heparin:

• Thrombotic event within last 18 months despite adequate anti-coagulationtherapy and/or

• Persistent thrombocytopenia and/or

• Persistent autoimmune hemolytic anemia

• Sex and Contraceptive /Barrier requirements for females.

Exclusion criteria:

SLE specific exclusion:

• Any acute, severe lupus related flare during the Screening Period that needsimmediate treatment

• Has any unstable or progressive manifestation of SLE

• Significant, likely irreversible organ damage related to SLE

RA specific exclusions:

• RA functional status class IV according to the ACR 1991 revised criteria

• Adult Juvenile RA

APS specific exclusions:

• Acute thrombosis (arterial or venous acute thrombosis diagnosis) less than 30 daysbefore study ISV

• Catastrophic APS classification within the prior 90 days of ISV