Leukaemia is a cancer of the blood. Most types of leukaemia cause the bone marrow to make abnormal white blood cells. These cells can get into the blood stream and circulate around the body. As these cells do not work normally they cannot protect you properly from infection.

Because there are far too many of these abnormal white blood cells, they can build up in the lymph nodes, bone marrow and spleen and cause swelling. They can also sometimes build up in the liver, the brain or the spinal cord.
Leukaemia affects the body by disrupting the normal balance of cells in the blood and their ability to do their jobs. Reduced levels of white blood cells, which normally defend the body from infection, can lead to higher chances of catching secondary diseases. Reduced levels of platelets, the cells that cause blood to clot, lead to more frequent bruises and nosebleeds. If the cancerous cells spread to the brain, headaches and nausea can result from tissues around the brain and spinal cord becoming irritated and inflamed. Finally, painful bones can result from the cancerous cells spreading into the bone marrow. (sharecare.com).

How leukaemia affects you
White blood cells help fight infection. But if your body makes abnormal white blood cells, they don’t work properly. So you are more likely to get infections and find it difficult to get rid of them.

Too many white blood cells can overcrowd the bone marrow. So, there is not enough space for other types of blood cells. Then you may have a lower than normal number of red blood cells and platelets.

Having too few red blood cells makes you tired and breathless (anaemic). And if you don’t have enough platelets, you can have bleeding problems (for example, nosebleeds).

Abnormal white blood cells can build up in parts of the lymphatic system, such as the spleen and lymph nodes, making them swell. They may build up in the liver. This can make your tummy (abdomen) swell and feel uncomfortable. The leukaemia cells can also spread to the brain, and the testicles in men.