Guess I'm a little more pro-active on things like this.
I know your dr. has probably ruled these things out???? But some causes of low WBC are bone marrow diseases or an enlarged spleen, pooling of the cells within the body (like in pyometra), HIV.
But my suggestion would be to do what YOU can do to boost your immune system B4 it bottoms out. There are many things you can take and you could ask your local health food store. But some that I know about are vit C, cat's claw (an herb - not actual cat's claws), astragulus, vit D, to name some off the top of my head.
Found this post and wondering if you got any answers concerning your dropping WBC. I seem to have similar issues. I have had low wbc for 30 years, 61 years old now. I saw a blood doctor 30 years ago and he ran all the tests except bone marrow test. He said I had unexplained leukopenia. Sister has slightly low count and brother does too, as well as my one daughter. None of them as low as mine. Which is 2.3 at present time. It was 2.7 thirty years ago.