This study is designed to evaluate the safety and tolerability of single ascending doses of UCB4019 administered by subcutaneous injection in healthy subjects.

Inclusion Criteria:

• Good physical and mental health
• At least 18 and less than 65 years of age
• Female subjects of childbearing potential must agree to use a highly effective method of birth control, during the study and for a period of 3 months after their last dose of study drug

Exclusion Criteria:

• Total Immunoglobulin G <7 g/L or >16 g/L at the Screening Visit
• Absolute neutrophil count <1.5x10^9/L and/or lymphocyte count <1.0x10^9/L
• Known viral hepatitis, has a positive test for Hepatitis B surface antigen or is Hepatitis C virus antibody positive
• Positive test to Human Immunodeficiency Virus (HIV) 1/2 antibodies
• Past medical history or family history of primary immunodeficiency
• Evidence of latent/active Tuberculosis (TB)
• Active infection or a serious infection within 6 weeks before the first dose of IMP
• Renal impairment
• Hepatic impairment
• Vaccination within 6 weeks before the Screening Visit or intent to have a vaccination before Day 43 of the Safety Follow-up Period
• Subject is splenectomized
• received any IMP or experimental procedure within 90 days before the first dose of IMP
• received UCB7665 in a clinical study