The suppressive immunological changes that follow surgical stress occur along a temporal pattern. Depression of cellular immunity to tumor-associated antigen and depression of delayed-type hypersensitivity are present for up to 3 weeks after surgery. In patients with pancreatic cancer the prevalence of regulatory T cells (Treg) is increased in the peripheral blood as well as in the tumour microenvironment. These Treg cells (CD4+25+) are phenotypically similar to Treg cells from normal donors by coexpression of CD45R0 and CTLA-4 molecules as well as cytokine profile. Regulatory CD4+25+ lymphocytes infiltrate tumor and regional lymph nodes, and suppress activation of CD8+ lymphocytes and helper CD4+25- lymphocytes (Liyanage, U. K., T. T. Moore, H. G. Joo, Y. Tanaka, V. Herrmann, G. Doherty, J. A. Drebin, S. M. Strasberg, T. J. Eberlein, P. S. Goedegebuure, and D. C. Linehan. 2002. Prevalence of regulatory T cells is increased in peripheral blood and tumor microenvironment of patients with pancreas or breast adenocarcinoma. J Immunol 169:2756).