In an immunisation programme for layer hens, both live and inactivated vaccines may be employed. Live vaccines stimulate the production of secretory immunoglobulin IgA and promote cellular immunity, mediated by CD8 + T lymphocytes, which are important to eliminate Salmonella settled in the cells. On the other hand, inactivated vaccines are related to the humoral immune response mediated primarily by circulating antibodies of the IgG group. It is important to point out that some of the serotypes used in live vaccines may lead to crossed responses to others that are non-homologous and belong to the same serogroup such as S. enteritidis and S. gallinarum.