Fig. 1.

Schematic of the pathways by which tumors block antitumor immune reactivity. Direct blockage of immune reactivity occurs through production of cytokines that are inhibitory to T_h1 antitumor responses. Tumor-derived cytokines can skew responses to T_h2 cytokine production, which also blocks T_h1 reactivity. Finally, tumors can mobilize immune inhibitory CD34⁺ precursor cells and block maturation of dendritic cells, both of which result in the failure to stimulate T_h1 antitumor activity