Although it was originally synthesised to induce experimental autoimmune encephalomyelitis, an animal model of multiple sclerosis, glatiramer acetate (GA) is actually used in the treatment of this human disease. Serendipity thus was responsible for the discovery of the therapeutic properties of what has become one of the only two first-line therapies currently approved for relapsing-remitting multiple sclerosis. Despite being discovered over forty years ago, novel aspects of the mechanism of action of GA are still being uncovered today. Initially, the immunomodulatory effects of GA were believed to involve high-affinity binding of the polypeptide to MHC Class II molecules on antigen-presenting cells. Subsequently, it was demonstrated that GA activated a specific population of GA-reactive T cells of a type-2 helper (Th2) phenotype, promoting an antiinflammatory environment and the preferential migration of GA-specific Th2 cells into the central nervous system, leading to decreased local inflammation through 'bystander suppression'. More recently, it has been shown that GA-reactive Th2 cells will secrete neurotrophins, important factors for neuronal survival and for axonal protection, in the central nervous system. Moreover, perhaps by this mechanism, GA increases proliferation, differentiation and survival of oligodendrocyte precursor cells; potentially enhancing myelin repair processes in situ. In parallel to this work, light has been shed on immunomodulatory effects of GA on other immune cell types. These findings were stimulated by the observation that adoptive transfer of GA-specific T cells alone had a limited capacity to suppress experimental autoimmune encephalomyelitis compared to injection of GA itself, suggesting that other cell types such as monocytes also played a role. It has now been documented that GA treatment can also modulate antigen-presenting cells such as monocytes, dendritic cells, and also additional adaptive immune system cell types such as CD8+ T cells and Treg cells. In this respect, it is important to note that the interplay between such antigen-presenting cells and T cells is fundamental given the coordinated and bidirectional interactions between these two cell types in the immune network.