Serum-depleted C5 inhibits NK-cell CD54 up-regulation, while serum-depleted C1q or C3 does not. PBMCs were mixed with Raji cells at a 1:1 ratio for 20 hours with 5 μg/mL of rituximab in the presence or absence of C1q-, C3-, or C5-depleted serum. Surface marker expression was determined using flow cytometry with gating on CD3⁻, CD56⁺ lymphocytes. NK-cell CD54, expressed as a percentage of CD54 bright, was determined. (A) Evaluation in C1q-depleted serum, C1q-depleted serum with purified C1q added back, or purified C1q alone (n = 3). (B) Evaluation in C3-depleted serum, C3-depleted serum with purified C3 added back, or purified C3 alone (n = 3). (C) Evaluation in various concentrations of C5-depleted serum (n = 3). Error bars represent SD of the mean.