Fig. 3: Mechanisms by which innate and adaptive immunity prevent and eradicate virus infections, copied from:(25).
It is reported that SARS-CoV-2 affect largely total lymphocytic count especially CD3, CD4 and CD8 cells which were found to be much lower in patients with severe clinical conditions and mortalities in relation to cured ones(29).
In fact, lymphocytopenia associated with change in the total leucocytic count is seen constantly in Covid-19 patients suggesting the disease severity and prognosis(19).
Moreover, in severe infections, flowcytometric analysis showed a significant decrease in all components of total leucocytic count including peripheral B lymphocytes, CD4 T helper lymphocytes, CD8 cytotoxic lymphocytes, NK cells, monocytes, eosinophils and basophils(67).
Another study done in Wuhan on 452 SARS-CoV-2 patients showed decrease in total count of T lymphocytes including both helper T cells, regulatory T cells (T reg) and memory T cells, however naïve T cells count was reported to be elevated(28).  Both naïve T cells and memory T cells are necessary to keep the immune response well-coordinated and effective as Naïve T cells are responsible of fighting new infections while memory T cells induce the antigen specific adaptive immunity. If there is an increase in naïve T cells in relation to regulatory T cells, then this favors an exaggerated production of pro-inflammatory cytokines leading to a hyperinflammatory response whereas if there is a decrease in the memory T cells count, this in turn could lead to reinfection with SARS-CoV-2 as these cells are in charge of defensing the body against the same infection in case of re-exposure(19). One of the explanations of decreased lymphocytic count in these patients is the direct infection of lymphocytes by SARS-CoV-2 leading to their lysis(68).
Regarding humoral immune system, Wen et al. showed marked decrease in naïve B lymphocytes count with significant increase of plasma cells count in the peripheral blood mononuclear cells of COVID-19 patients during their recovery period. Furthermore, some modifications in B cell receptors were noticed(69). As B cells play a significant role in controlling viral infections, monitoring the seroconversion patterns in COVID-19 patients is crucial for assessing their conditions clinically. It was reported that 96.8% of patients had seroconversion of IgG or IgM within 20 days from the onset of symptoms followed by a plateau phase for 6 days later. All the studied subjects had virus specific IgG antibodies around 17-19 days since the first symptoms appeared while only 94.1% of them achieved virus specific IgM antibodies around 20-22 days after the start of symptoms(70).