How do tumor cells metastasize?

Tumor cells metastasize primarily by breaking off from the original (primary) tumor and traveling through either the blood or lymphatic systems. This process allows cancer cells to spread to distant organs and tissues, establishing secondary tumors. The ability of cancer cells to detach from the primary site and invade surrounding tissues is a key characteristic of their malignancy.

Once in the bloodstream or lymphatic system, these cells can circulate throughout the body. When they reach a new site, they may adhere to the blood vessel walls, exit the circulation, and proliferate in that new environment, leading to the formation of metastases. This mechanism is crucial for understanding the progression and treatment of cancer, as the spread of cancer is often a significant factor in patient outcomes.

The other options do not accurately represent the biological mechanisms of metastasis. For instance, tumor cells do not remain stagnant; they actively seek routes to spread. Additionally, while immune cells can kill some cancer cells, they are not typically effective enough in preventing metastasis in many cases, especially as tumors often develop strategies to evade immune detection. Lastly, tumor cells do not disperse via airborne particles, as their spread primarily relies on direct invasion into the circulatory and lymphatic systems.