This paper analyzes violence from the peculiar perspective of a “social immunology”, defined as the area of social systems theory concerned primarily, even though not exclusively, with those processes by which social systems as communication systems deal with tensions, termed ecological tensions, that involve the intersection between social systems and their human environment and that endanger the maintenance of social autopoiesis. Individuals’ violent behaviors are manifestations of such tensions. However empirically ubiquitous it may be, violence constitutes a crucial problem for social systems, due to risks detrimental to the human substrate of communicative processes. We theorize about conflicts, social movements and the legal system as the main components of an immune apparatus of social systems by focusing on individuals’ reactions to social costs and related social conflicts. Modern law, in particular, tends to promote individuals’ inclusion and to restrain their resorting to violence by relying on a system of rights, procedures, and judicial institutions conducive to attracting and regulating such conflicts. Legal immunization remains nonetheless exposed to several limitations, including abnormal forms that concur to the arbitrary use of violence.