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).