2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/122951We discuss the impact of magnetic field on the mass - temperature relation for groups and clusters of galaxies based on the derivation of the general Magnetic Virial Theorem. The presence of a magnetic field B yields a decrease of the virial temperature T for a fixed mass M: such a decrease in T is stronger for low-mass systems than for high-mass systems. We outline several implications of the presence of B-field and of its mass scaling for the structure and evolution of groups and clusters.5 pages, 2 figures, published in A&A Letters, references updated, details added in the text and in the AppendixAstrophysicstext