This study investigates whether the probiotic Lactobacillus plantarum can prevent gastrointestinal graft-versus-host disease (GI aGvHD) in children, adolescents, and young adults who are undergoing an alternative allogenic hematopoietic cell transplant (alloHCT). GI aGvHD is a condition where the donated cells attack the gastrointestinal tract of the recipient, causing inflammation and damage.

Participants will be given either the probiotic Lactobacillus plantarum or a placebo. A placebo is an inactive substance that looks like the investigational medicine but does not contain any medicine. The study will monitor the development of GI aGvHD and other health outcomes in the participants.

• Who can participate: Children, adolescents, and young adults undergoing an alternative donor allogenic hematopoietic cell transplant can participate. Participants must not have a history of previous hematopoietic cell transplant or cellular therapy and must be receiving cells from a first alternative donor.
• Study details: Participants will receive either the probiotic Lactobacillus plantarum or a placebo. The study will track the health and development of GI aGvHD in participants.