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 initial screening 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 Against Rheumatism (EULAR) or American College of Rheumatology (ACR) Classification Criteria
- Positive anti-double stranded deoxyribonucleic acid (anti-dsDNA) autoantibody and/or positive 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 >=6 months 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 >=3/66 Swollen 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/IgM anticardiolipin (aCL) or moderate to high titers of IgG/IgM anti-beta2-glycoprotein I antibody using central lab test, at ISV
- Clinical features attributable to antiphospholipid antibodies that are resistant to warfarin and/or heparin:
- Thrombotic event within last 18 months despite adequate anti-coagulation therapy 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 needs immediate 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 days before study ISV
- Catastrophic APS classification within the prior 90 days of ISV