In degenerative conditions such as AD, ALS, Huntington's disease (HD), and Parkinson's disease (PD), overactivation of components like C1q, C3, and C5a drives aberrant synaptic pruning, fuels microglia-mediated neuroinflammation, and in some cases induces dopaminergic neuron loss. This evidence concerns the gene C5AR1 and Parkinson disease.