It has been confirmed that both the primary tumor and secondary tumor in mouse models of breast cancer, in their tumor microenvironment, Th2 cells can promote the metastasis of lung mesenchymal stromal cells (LMSCs) by producing IL-4 and IL-13, and the LMSCs can produce complement C3 to act on neutrophils to recruit neutrophils migration into the blood stream, causing NETs formation, which promote metastasis of tumor cells to the lung, and increased C3 levels can also be detected in sera of 41 breast cancer patients from the First Affiliated Hospital of Soochow University [65]. Here, C3 is linked to breast cancer.