Study to Test the Safety, Tolerability and Efficacy of UCB7665 in Subjects With Moderate to Severe Myasthenia Gravis

Inclusion Criteria: * Subject has a well-documented diagnosis of myasthenia gravis (MG) at Visit 1 (Screening), based on subject history and supported by previous evaluations * Subject would currently be considered for treatment with immunological therapy (immunoglobulin/plasma exchange (IVIG/PLEX)) by the investigator * Subject has a well-documented record of autoantibodies against anti-acetylcholine receptor (Anti-AChR) or anti-muscle specific kinase (Anti-MuSK) prior to Screening * Female subjects must either be: postmenopausal, permanently sterilized or if childbearing potential applicable will use a highly effective method of birth control * Male subjects must be willing to use a method of contraception Exclusion Criteria: * Subject has previously received treatment in this study or subject has previously been exposed to UCB7665 * Subject has participated in another study of an investigational medicinal product (IMP; or a medical device) within the previous 30 days of Screening or is currently participating in another study of an investigational medicinal product (IMP; or a medical device) * Subject has a known hypersensitivity to any components of the IMP * Subject has a history of hyperprolinemia, since L-proline is a constituent of the UCB7665 IMP * Subjects with Myasthenia Gravis (MG) only affecting the ocular muscles * Subjects with severe weakness affecting oropharyngeal or respiratory muscles, or who have myasthenic crisis at Screening or impending crisis * Subject has quantitative myasthenia gravis (QMG) score of \<11 at Baseline * Subject has a serum total immunoglobulin G (IgG) level \<= 6g/L at Screening * Absolute neutrophil count \<1500 cells/mm\^3 * Subject has any medical condition (acute or chronic illness) or psychiatric condition that, in the opinion of the investigator, could jeopardize or would compromise the subject's ability to participate in this study * Subject has any laboratory abnormality that, in the opinion of the investigator, is clinically significant, has not resolved at randomization, and could jeopardize or would compromise the subject's ability to participate in this study * Subject has received a live vaccination within 8 weeks prior to the Baseline Visit; or intends to have a live vaccination during the course of the study or within 7 weeks following the final dose of IMP * Subject has received any experimental biological agent within or outside of a clinical study in the past 3 months or within 5 half-lives prior to Baseline (whichever is longer)