The ability of tegoprubart (formerly known as AT-1501) to block immunoglobulin class switching in healthy subjects who have received a Keyhole Limpet Hemocyanin (“KLH”) challenge was explored in a subset of the 8 mg/kg cohort of the Phase 1 safety study where KLH, a shellfish protein, was injected subcutaneously in four subjects. In the control subject that did not receive Tegoprubart, there was a robust antibody response to the foreign KLH protein which peaked at 15 days. Tegoprubart, however, successfully completely abrogated an immune response to KLH in two of the three treated subjects and reduced the immune response in the third treated subject.