BTK and B-cell chronic lymphocytic leukemia: Through irreversibly binding to the cysteine residue (C481) in Bruton's tyrosine kinase (BTK) domain and inhibiting its enzymatic activity, covalent BTKi interferes with B-cell receptor signaling, affecting adhesion, migration, proliferation and cell survival, resulting in redistribution of CLL cells from secondary lymphoid organs to the PB, reducing lymphadenopathy and splenomegaly, with an expected transient increase in PB lymphocytes over the first weeks or months of treatment.