The altered metabolism of many cancers causes malignat cells to make large amounts of lactic acid. This reflects a phenomenon known as “aerobic glycolysis”, in which cancer cells take up large amounts of glucose (blood sugar), convert it to lactic acid, and excrete it; the excreted lactic acid makes the extracellular environment of tumors acidic, an effect which is often potentiated by poor blood flow in some tumor regions.
Certain regulatory changes which cause cancers to behave aggressively tend to promote aerobic glycolysis; for this reason, aggressive cancers tend to be more acidic. But recent medical studies reveal that extracellular acidity itself tends to make cancer cells behave more aggressively; hence, if cancer cells are incubated at a mildly acidic pH, and then injected into mice, they are more prone to produce metastases. Why acidity has this effect is still not clear.
The high extracellular lactic acid content of many cancers also protects them from immune cells (cytotoxic T lymphocytes and natural killer cells) capable of killing the cancer; within the acidic tumor environment, these immune cells tend to soak up the lactic acid, and this impairs their protective activity. Hence, extracellular acidity makes cancer cells more dangerous in at least two ways – it makes the cancer cells more aggressive and capable of spreading, and it protects the cells from immune rejection.