C4A and hereditary spastic paraplegia: In HSP rabbits, the serum IgA levels (0.084 ± 0.0005 vs. 0.04 ± 0.00003 g/L) (P < 0.05) significantly increased compared to the controls, while the IgG (0.44 ± 0.09 vs. 0.53 ± 0.06 g/L) (P < 0.05), C3 (0.13 ± 0.048 vs. 0.21 ± 0.035 g/L) (P < 0.01), and C4 (0.02 ± 0.004 vs. 0.05 ± 0.010 g/L) (P < 0.01) levels significantly decreased, and there was no change in serum IgE levels (0.011 ± 0.0067 vs. 0.029 ± 0.0057 g/L) compared to the controls (Fig. 2D).