Schizophrenia is a devastating mental illness characterized by a broad range of clinical manifestations including hallucinations, social cognitive impairments, and disordered thinking and behavior, all of which impair daily functioning. The immune molecule complement component 4 (C4), located in the major histocompatibility locus (MHC) on chromosome 6 in humans, is highly associated with schizophrenia such that specific structural variants and regulatory regions increase the expression of C4 and confer greater risk for this brain disorder. Besides their established role in brain immune defense, complement proteins play a role in various stages of brain development including neurogenesis, migration and synaptic development. However, C4 has never been experimentally upregulated to determine the impact of increased expression of this immune gene on brain development. Here, I study the role of C4 in layer 2/3 pyramidal neurons in the medial prefrontal cortex of mice to study the hypothesis that C4 overexpression causes circuit dysfunction by leading to the pathological elimination of synapses. Specifically, neuronal connectivity was assayed by measuring dendritic spine density using confocal microscopy and functional connectivity through whole-cell electrophysiology recordings. Additionally, the role of microglia in altering the developmental wiring of the brain was examined by quantifying microglia engulfment in the medial prefrontal cortex. Lastly, complement-induced changes to the prefrontal cortex were accompanied by deficits in social behavior in both juvenile and adult mice. Overall, these studies show that C4 affects brain connectivity by reducing dendritic spine density and excitatory drive through enhanced microglia-engulfment of synaptic material which was sufficient to cause lasting deficits in mouse social behavior.