Experimentally aged erythrocytes are phagocytosed by macrophages after 4N1K binding. (A) Aging of fresh erythrocytes in combination with 4N1K peptide incubation induces phagocytosis by differentiated THP-1 cells (n = 3). (B) Phase-contrast image of primary human red pulp macrophages. Arrowheads indicate the erythrocytes still associated with the red pulp macrophages on isolation. (C) Expression of SIRPα on primary human red pulp macrophages, as detected by flow cytometry. (D) Aging of fresh erythrocytes in combination with 4N1K peptide incubation induces phagocytosis by primary human red pulp macrophages (n = 2). ns indicates not significant. *P < .05; **P < .01; ***P < .001.