Study of XmAb942 in Healthy Participants and Participants With Ulcerative Colitis

Who May Qualify: Parts A and B - Age 18-55 - Must be in good health with no significant medical history - Clinical laboratory values within normal range - BMI 18-35 (inclusive) - Contraceptive use by men or women consistent with local regulations - Able and willing to provide written willing to sign a consent form Part C - Age 18-75 - Must be in good health with no significant medical history - UC diagnosis ≥ 3 months prior to screening - Diagnosis of moderately to severely active UC as defined by a (MMS) ≥ 5, with a MES ≥ 2 and RBS ≥ 1 - Evidence of UC extending ≥ 15 cm from the anal verge, as determined by screening colonoscopy - Must have inadequate response to, loss of response to, or intolerance to at least 1 of the conventional or advanced therapies of UC - Able and willing to provide written willing to sign a consent form Who Should NOT Join This Trial: Parts A and B - Any physical or psychological condition that prohibits study completion - History of suicidal behavior or suicidal ideation - Heavy use of nicotine containing products - HIV, hepatitis B and hepatitis C positive - Cardiac arrhythmia, or clinically significant abnormal ECG - Active use of prescription medications within 14 days of Day -1 - Active use of over-the-counter, or herbal medication within 7 days of Screening - Other investigational products within 30 days - Blood or plasma donation within 60 days - Pregnant or breastfeeding Part C - Any physical or psychological condition that prohibits study participation - Diagnosis of Crohn disease, indeterminate colitis, indeterminate colitis, microscopic colitis, ischemic colitis, infectious colitis, radiation colitis, and diverticular disease associated with colitis. - Positive screen for Clostridium difficile (C. Difficile) toxins - HIV, hepatitis B and hepatitis C positive - Cardiac arrhythmia, or clinically significant abnormal ECG - Pregnant or breastfeeding Other protocol defined inclusion/exclusion criteria apply. Always talk to your doctor about whether this trial is right for you.