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Long-term use can also cause a deficiency of vitamin B-12, which leads to reduced white blood cell count and anemia.

These take instructions from the white blood cells known as dendritic cells, which he refers to as the "generals" of the immune system.

When people donate blood, stem cells in the bone marrow make new blood cells to replace the lost blood - and this could shape the stem cells' genetic diversity.

Untreated, the clots cause organ damage and low red blood cell and platelet counts.

Part of our immune defences - a white blood cell called a T-cell - can swoop in and destroy the spreading cancer as it tries to take root.