Fig. 1.

Immune cells (including T cells, DCs, NKs and others) can defend against primary tumors (A) or metastatic lesions (B). Immune cell infiltrates can differ in composition in primary tumors versus metastases, in terms of numbers, types, activation levels, and/or specificities (A, B). In some instances, immune cells can potentially modify tumor heterogeneity by selective targeting (C). Immune cell infiltration can be variable in location within tumors, between tumors, and/or can change in quantity or composition within the same tumors over time (D).