I have heard that leukimia(Blood cancer) increases white blood cells(I don't know the type).White blood cells are meant for defence mechanism isn't.Then why is it bad????

I think this is reasonably well established:

 

Leukemia cells

 

In people with leukemia, the bone marrow produces abnormal white blood cells. The abnormal cells are leukemia cells. At first, leukemia cells function almost normally. In time, they may crowd out normal white blood cells, red blood cells, and platelets. This makes it hard for blood to do its work.

 

 

You can learn more here: http://www.medicinenet.com/leukemia/article.htm

leukemia is the production of the lymphocytes either B cell or T cells, B cells are more common as they are not as dangerous as T cell leukemias, Adult T cell leukemias are able to kill in the space of just a few months where as people can suffer from chronic lymphocytic leukemias (B cell) for years. It is a marked increase in lymphocytes, and are at most time anergic thus do not perform their immune role. There is a fault in the selection and survival signals which are received by lymphocytes during selection which inhibit the release of anergic and autoreactive lymphocytes