What is Blood Cancer? Thinking about how cancer forms, we know that our bodies are always making new cells. With the production of cells come mutations. Some mutations cause the cells in our body to multiply without a purpose. Blood cancer occurs when those mutations replicate at a large scale in the blood, producing an excess of cells. In our blood, we have white blood cells, red blood cells and platelets. The white blood cells fight infections. Red blood cells take oxygen to our tissues and organs, wh...