Background: Alzheimer's disease (AD) is characterized by the dynamic accumulation of extracellular amyloid deposits from the interplay between amyloid-β (Aβ) plaques, reactive astrocytes and activated microglia. Several immunotherapies against Aβ have been shown to reduce amyloid neuropathology. However, the role of the associated glia in the recovery process requires clarification. Previously, we described the safety and effectiveness in aged domestic canine with cognitive dysfunction syndrome of a new active vaccine candidate for the treatment of AD in humans. Objective: The aim of this article is to gain a better understanding of how immunotherapy modifies the amyloid burden and its effects on astroglial and microglial reactivity in immunized dogs. Methods: In order to achieve this, we compared and quantified amyloid plaques and astroglial and microglial reactions in the frontal cortex of unimmunized and immunized aged domestic dogs. Results: We found amyloid plaques from immunized dogs to be smaller and more compact than those from unimmunized dogs. In these new plaques, the associated astrocytes were closer and less immunoreactive to the β subunit of S100 protein (S100B). We also found no modification in the microglial reaction associated with immunization. Conclusion: The anti-Aβ immunotherapy developed in our laboratory modifies the equilibrium between soluble and insoluble Aβ in aged dogs in close correlation with S100B-negative astrocytosis and microglial reaction.