Generally, low white blood cells count has many possible causes which include different leukemias, virus infections, as side effects of some medications amongst others.
Low WBC may be due to a virus infection that does not need so much high concern but it can also include sorts of serious viral infection such as HIV infection. Low WBC can also be due to different types of leukemia. Some medications such as Chloramphenicol are also known to cause low WBC count which is usually reversible. TO be sure that the cause is not a virus infection, WBC count test needs to be repeated. There is a little increase in your count so let us hope for more changes in the next test. Due to low WBC count, your immune system cannot perform well its function to protect the body from any infection. The low grade fever you have is one of the signals of a setting in infection. Easy bruising is one of the conseqences of any blood disorder. I am quite concerned with the weight liss though.
In cases on low WBC, peripheral smear are usually helpful to help seefor any presence of any leukemia. Your peripheral smears should immature lymphocytes - that is one of the reasons why your WBC is low because cells fail to complete maturity for some reasons.
I would think of doing tasts such as ANA tests to determine the presence of neutrophil antibodies which can indicate presence of collagen vascular disease. I would also think of doing Immunoglobulin tests to check for any defects in cellular immunity. I would also consider tests for Vitamin B12 to check for possibility of Pernicious anemia.
If the subsequent repeat test won't show any improvement of WBC especially if there is further lowering of the level and your condition gets worse, doing a bone marrow biopsy would be a definitive test that can lead to your diagnosis.
For the meantime, please try to avoid crowd and try to stay away from any possible source of infection. Your immune system is down and at this time, it will be very easy for you to catch any sorts of infection. Take care!
If your WBC goes up and stays within normal limits, I would say the cause of a low WBC was reversible thus it should not be a cause of high concerns. As you mentioned you have swollen lymph node in neck and have family history of non-hodgkin's (though there is no hereditary pattern established in non-hodgkins), I would insist on doing bone marrow biopsy as well as lymph node biopsy and also consider imaging tests either CT Scans, MRI or PET.
If the biopsies and imagig finds nothing, then it would be a good news and will end up your worries. If it finds something, will still be at your advantage. Earlier diagnosis will give a greater chance to have a successful treatment. With early diagnosis, leukemia and non-hodgkin's survival rate became higher.
With your condition, I understand how concerned you are. You're right! Your condition needs etensive follow up to arrive to a definite diagnosis.
So you have done MRI as well, we should wait what the result says then. The lymphocytes levels in leukemia are not always high. It is also possible to have low levels of lymphocytes count in leukemia. Now, don't think that I am saying you have leukemia. You need series and follow ups of investigation to arrive to diagnosis. There are many other possible causes that can affect the lymphocytes too which include use of corticosteroids, autoimmune diseases, infection or disorders of the lymphoid tissue in the spleen, lymph nodes, and thymus gland. Your should have immune tests as well. You have not mentioned bone marrow biopsy, if not done yet, I would highly advise to have it done.
As far as I know, Celiac does not directly cause low lymphocytes count but what I know is that in Celiac, the epithelium villi of the intestines are infiltrated with lymphocytes. But I think, they are running Celiac test to establish some association to your current condition.
Lastly I would say, NOT ALL causes of low white blood count or low lymphocytes are known in our current time. There are many cases similar to you that run all sorts of tests but found nothing and are therefore categorized as IDIOPATHIC for the time being, which means the condition is with UNKNOWN cause.
For now, bear with your doctors who are trying to find leads to your problem.