Table 10.

Possible Mechanisms of Tumor Escape from Immune Destruction

Lymphocyte factors

Lack of T-cell help

Insufficient number of antitumor T cells

Insufficient avidity of T cells for tumor

T cells are “tolerized”

Down-regulation of T-cell receptor signal transduction

Apoptosis of T cells when encountering tumor

Inadequate T-cell function (cytokines, lysis)

T cells cannot enter tumor stroma

Inhibition by “suppressor” T cells

Tumor factors

Tumor cannot activate quiescent precursors

Insufficient tumor antigen expression

Loss of HLA expression by tumor

Tumor produces local immunosuppressive factors

Tumor lacks sufficient apoptotic or other cell destruction

pathways