Complex sentence: After leaving the bone marrow, the B cell acts as an antigen presenting cell (APC) and internalizes offending antigens, which are taken up by the B cell through receptor-mediated endocytosis and processed.
Split into simpler sentences: After leaving the bone marrow, the B cell acts as an antigen presenting cell (APC) and internalizes offending antigens. That antigen is taken up by the B cell through receptor-mediated endocytosis and processed.