Inhibitory effect of serum is dose dependent and abrogated by heat inactivation. PBMCs and Raji cells were mixed at a 1:1 ratio for 20 hours in varying concentrations of serum or heat-inactivated serum in the presence of 0.2 μg/mL rituximab. Surface marker expression was determined using flow cytometry with gating on CD3⁻, CD56⁺ lymphocytes. (A) NK-cell CD16, expressed as median fluorescence, after incubation with varying concentrations of serum and heat-inactivated serum (n = 3). (B) NK-cell CD54, expressed as a percentage of CD54 bright, after incubation with varying concentrations of serum and heat inactivated serum (n = 3). Error bars represent SD of the mean.