Discussion:
Due to the semi-allograft nature of the fetus in utero, the immune response requires both stimulatory and suppressive functions for a successful pregnancy outcome. Lymphocytes are a contributor in the maternal immunological response to pregnancy. The normal lymphocyte count changes in pregnancy, and there is no consensus in the literature about how it changes. Some older papers suggest that there was no change overall in lymphocyte count during pregnancy (13,14). Results from more recent studies suggest that there is a decrease in lymphocyte count (15,16), with the biggest decrease occurring in the first and second trimesters (5). The bulk of available literature agrees with the findings of a reduction in overall lymphocyte count; however, a recent study in 2013 found a significant increase in the lymphocyte count of 200 pregnant women (17,18), further complicating the discussion.
Some papers suggested lymphopenia is associated with worse outcomes in COVID patients. (4) Another study observed that, progressive decline in the lymphocyte count and rise in the D-dimer over time were observed in non survivors compared with higher levels in survivors after COVID-19 infections (5).The same authors observed that common laboratory findings among hospitalized patients with COVID-19 include lymphopenia, elevated aminotransaminase levels, elevated lactate dehydrogenase levels, and elevated inflammatory markers (e.g., ferritin, C-reactive protein, and erythrocyte sedimentation rate) (5)
In a series of 393 adult patients hospitalized with COVID-19 in New York , 90 % had a lymphocyte count <1500/microL; leukocytosis (>10,000/microL) and leukopenia (<4000/microL) in 15% of cases(6)
Another study showed that high D-dimer levels and marked lymphopenia, are associated with higher mortality rate. (7) A retrospective analysis of eight pregnant cases of SARS-CoV-2 showed that 50% were asymptomatic before delivery but became symptomatic post-partum. This was associated with ,raised white blood cell counts , and reduced lymphocyte counts. C-reactive protein levels increased (8).
In another contradicting cohort study of 55 patients who gave birth  with suspected COVID -19 disease of which 13 later turned out positive to COVID-19, there was no difference in the WBC, the ratio of the neutrophils and lymphocytes, CRP between both suspected and confirmed cases groups.(9)
Another study observed that in COVID-19 positive cases there was abnormalities on chest imaging, lymphopenia, leukopenia, and thrombocytopenia. (10)
A meta-analysis of 33 studies reporting 385 pregnant women with COVID-19 infection: noted 0.8% critical, 3.6% severe infection, 95.6% with mild symptoms. There were 6 women admitted to intensive care and one maternal mortality. Laboratory findings included elevated D-dimer in (22.3%), elevated C-reactive protein in (18.7%), lymphopenia in (14.0%), and thrombocytopenia in (1.0%) women. (11)
In another meta-analysis, lymphopenia was found in 79.8% (40/48) of cases. Also, it was reported that radiological features suggestive for pneumonia could be found in almost all of the hospitalized pregnant women, usually presenting with fever, cough and lymphopenia similar to the non-pregnant population. (12)
A recent meta-analysis with 828 patients, where 407 patients had severe disease (49.15%) concluded that the NLR values were found to increase significantly in patients with COVID‐19 with severe disease (SMD = 2.404, 95% CI = 0.98‐3.82) Increased NLR levels reflect an enhanced inflammatory process and may suggest a poor prognosis. (20).
Coronavirus mainly act on lymphocytes, especially T lymphocytes. Surveillance of NLR and lymphocyte subsets helps in screening for critical cases of COVID-19. A trial with 452 patients with COVID-19 , 286 were diagnosed with severe infection. Severe cases tend to have lower lymphocyte counts, and neutrophil-lymphocyte ratio (NLR). The number of T cells was significantly reduced in severe cases. Both helper and suppressor T cells in patients with COVID-19 were below normal levels, with lower levels of Th cells in the severe group. Patients with COVID-19 also have lower levels of regulatory T cells, which are more reduced in severe cases. (21)
Neutrophil to lymphocyte ratio (NLR), have been recognized as systemic inflammatory response (SIR) marker for many years. (22). Research revealed that NLR offer effective indication of severity of diseases as pre-eclampsia. NLR, with a cutoff value of 5.6, predicted severe pre-eclapmsia with sensitivity 93.3% and specificity 86.6% . (23)
Another meta-analysis with 1371 patients showed significantly higher NLR in non-survivors than in survivors. They also looked at 10,685 patients evaluating the value of NLR for sepsis prognosis; results showed that higher NLR was associated with poor prognosis in patients with sepsis . (24)
Another study looked at NLR as a predictor of mortality and treatment response in sepsis patients in the intensive care unit (ICU). They looked at NLR on the 1st, 3rd, and last day in the ICU. If the NLR was ≥15 on the 3rd day, the mortality odds ratio was 6.96 (25)