How does these tow effects.a decrease in white blood cells and increase in susceptibility,related?

Answer 1

White blood cells (WBC) are cells that fight infection. They are very important in our immune system.

If they are decreased, then our ability to fight infections is also decreased.

Oddly, there are some illnesses where their numbers are INCREASED, but what actually happens is lots of useless WBC are produced and fewer useful WBC are made so people with these types of cancers can have susceptibility to illness (esp infections) too.

Things that decrease your WBC:
o Drugs - especially chemotherapy drugs (some recreational drugs can rarely do this)
o Cancers - by growing in the bone marrow and crowding out the cells that make RBC and WBC, it is possible to cause decreased numbers of WBC
o Infections - in severe infections where the bacteria are killing the patient's WBC and the WBC count is therefore low, this is the picture of quite a nasty and overwhelming infection.

Answer 2

A high white blood cell count is usually a sign of infection. Only a doctor should determine the cause and suggest treatment.