IgG subclasses induced by the SPIONs/PEI/DNA + HA complexes in vivo. BALB/c mice (n = 5/group) were immunised 3 times (3 weeks apart) with either SPIONs/PEI/DNA + HA, SPIONs/PEI/DNA, or naked DNA, via i.p. and i.m. administration (naked DNA via i.p. only), with or without the application of an external magnetic field. Two weeks after the final immunisation (day 56), sera from each mouse were collected and measured for antigen-specific IgG1 (A), IgG2a (B), and IgG2b (C) production by ELISA assay. Data represented as antibody titre mean ± SD (n = 5 mice). Statistical analysis was performed using one-way analysis of variance (ANOVA) and Tukey’s multiple comparison tests. Statistical significance was designated as ** p < 0.01, *** p < 0.001, **** p < 0.0001, ((w/M) with magnet, (w/o M) without magnet).