B cell development trait B cell morphology trait immature B cell morphology trait Trait.ontology These cells express IgM on their surface, in addition to CD45R, MHC class II, CD19, CD20, CD21, CD40. immature B lymphocyte morphology trait VT:0005022 immature B-cell morphology trait Any measurable or observable characteristic related to the shape, structure, color, or pattern of the cells of the B lymphocyte lineage that have undergone VDJ rearrangement of the heavy chain and V-J rearrangement of the light chain; these cells express IgM on the cell surface but have not yet been selected for self-reactivity.