**6. Hematologic system and COVID-19 infection**

The COVID-19 infection has significant impact on hematopoietic system like other viral infections such as varicella, dengue, MERS-CoV, etc [25–33]. The most common haematological changes observed are lymphopenia, neutrophilia and eosinopenia [25]. It has been found that lymphopenia, thrombocytopenia and leucocytosis have been associated with increased severity and fatality in COVID-19 cases [26]. The ACE2 receptor is expressed on lymphocytes and this virus directly infects lymphocytes causing cell lysis [27]. Also the cytokine storm promotes the lymphocyte apoptosis. It has been recommended that the serial assessment of lymphocyte count must be ensued as an indicator of prognostic outcome [28].

The changes in haemostasis tests like prolonged prothrombin time, activated partial thromboplastin time and elevated D-Dimer levels has been found during the COVID-19 infection [29]. Increasing D-dimer levels and formation of microthrombi in peripheral blood vessels have been associated with severe forms of COVID-19 infection [30]. Also increased ESR, CRP and Serum LDH has been found. Liu et al reported that the severity of COVID-19 infection can be predicted by lymphopenia, neutrophilia and high levels of CRP and Serum LDH [31]. These altered coagulation profiles also suggest that this virus stimulates a low grade DIC state and resulting thrombocytopenia due to consumption [32]. Few researchers also believe that virus infect bone marrow hematopoietic cells directly inducing growth inhibition and apoptosis [33].
