AbstractImmunoglobulin G (IgG) is one of the five antibody classes produced in mammals as part of the humoral responses. This high-affinity antibody produced late in a primary immune response is responsible for protecting the organisms from infection. This protein’s heavy chain constant region is encoded by the Ig gamma gene (Igγ). In Humans, IgG has evolved into four subclasses with specialized effector functions. However, in Platyrrhini, IgG has been reported to be encoded by a single-copy gene. Here, we analyzed data from 38 primate genome sequences to identify Igγ genes and describe the evolution of this immunoglobulin in this group. Igγ belongs to a multigene family that evolves by the birth-death evolutionary model in primates. Whereas Strepsirrhini and Platyrrhini have a single-copy gene, in Catarrhini species it has expanded to having several paralogs in their genomes; some have been deleted or have become pseudogenes. Furthermore, episodic positive selection might promote a specialized species-specific IgG effector function. A hypothesis for the Igγ evolution has been proposed, suggesting that IgG has evolved to reach an optimal number of copies per genome to adapt their humoral immune responses to different environmental conditions.Graphical abstract