Mechanisms of CD19-targeted immune escape. The B-cell coreceptor complex consists of CD19, CD21, CD81, and CD225 (for simplicity CD225 is omitted here). It is assembled in the rough endoplasmic reticulum (rER) and transported to the Golgi apparatus where the carbohydrate moieties are processed before being transported to the cell surface. Carbohydrate processing is indicated only for CD19 (light red: unprocessed carbohydrates; dark red: processed carbohydrates). (A) Normal processing and transport of CD19. (B) Splice variants resulting in loss of the extracellular domain of CD19. (C) Mutations resulting in a conformational change of the extracellular domain of CD19. (D) Loss of CD81 expression resulting in intracellular accumulation of CD19 with unprocessed carbohydrates. (E) Lineage switch resulting in transcriptional silencing of CD19 expression.