Figure 2: HLA class II-restricted helper CD4+ T cell responses can prolong CD8+ CTL activity. (A) CD8+ T cells recognize HLA class I-peptide complexes on tumor cells and directly kill cancer cells, reducing tumor burden. However, this reaction is not sustained for very long and the remaining tumor mass may continue to divide and progress. (B) Tumor cells that express HLA class II proteins or are induced to express class II proteins, can stimulate CD4+ T cells resulting in prolonged CD8+ CTL response and sustained tumor killing through the release of cytokines and immune stimulatory molecules.