IL-2 enhances the proliferation of Tregs within the tumor [42] Representing this chemoattraction by ∇⋅(χPTr∇P), we get the following equation for Tr:∂Tr∂t+∇·(uTr)-δT∇2Tr=λTrTβT10TβKTβ+Tβ}Tβ-enhancednaiveTcellsactivation+λT1TrT1QKQ+Q}Q-inducedT1→Trtransition+λTrI2TrI2KI2+I2}IL-2-inducedproliferation-∇·(χPTr∇P)}chemoattractionbyCCL2/MCP-1-μTrTr}death,(6)where the second term in the right-hand side is the same as in Eq (4). Here, CCL2 is linked to neoplasm.