Patients with advanced stage chronic lymphocytic leukemia, a B-cell neoplasm, were treated with anti-CD20 Fab fragment bound to a radioactive molecule for directed killing. Surprisingly, use of an intact anti-CD20 molecule without attached toxin or radioactive molecule was found to be as effective as the radioactive preparation with less toxicity on normal cells (radioactivity killed surrounding normal hematopoietic cells in the bone marrow). The intact immunoglobulin was needed to do which of the following?

a. Bind more effectively to the tumor cells.

b. Provide Fc biological properties (complement binding; opsonization).

c. Allow targeting by CTLs.

d. Block CTLA4 on T cells.

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