CD4 and cancer: However, there are also considerable differences in chronic viral infections and cancer that potentially impact the CD8+ T cell response: e.g., origin of antigen (exogenous versus endogenous) associated with antigen quantity and presentation, the differential expression of cytokines and other immune mediators (pro-inflammatory versus immunosuppressive), and the composition of the other immune cells (CD4+ T cell help, CD4+ regulatory T cells, myeloid-derived suppressor cells, and tumor-associated macrophages).