This study is researching a drug called pozelimab (called "study drug"). The main aim of this study is to monitor the safety and tolerability of the study drug. The study is focused on young children 1 to 5 years of age, who have CHAPLE disease. CHAPLE is a very rare hereditary disease that can cause potentially life-threatening symptoms related to the stomach and intestines (gastrointestinal symptoms), and symptoms related to the heart and blood vessels (cardiovascular symptoms). The study is also looking at several other research questions, including: * What side effects may happen from taking the study drug * How much study drug is in the blood at different times * Whether the study drug blocks Complement 5 (C5) in the body * Whether the study drug changes the level of a substance called CH50 measured in the blood * Whether the study drug changes the levels of albumin and other proteins * Whether the body makes antibodies against study drug, which could make the study drug less effective or could lead to side effects