Phenotypic features of adaptive immune system cells in COVID-19
A representative figure for flow cytometric evaluation of lymphocyte subsets was shown in Figure 4. All percentages and the absolute numbers of lymphocyte subsets was given in Table 2.
The percentage and the absolute number of B lymphocytes were extremely low in all groups of COVID-19 patients:  The percentage of B lymphocytes evaluated by surface CD19 was low in all COVID-19 patients compared to control (p <0.01, Figure 4b, Table 2). There was no difference among patient groups. Furthermore, when the absolute B lymphocyte counts were examined, the number of B lymphocytes was found to be remarkably decreased in patients with COVID-19 (p<0.02, Table 2). In the severe group, the number of B lymphocytes decreased up to approximately one-fifth of the control. CD19 and CD138 were stained in a few patients with the thought that the reason for this extreme reduction may be the result of the differentiation of B lymphocytes into plasma cells. However, no CD138 positive plasmablast was found in the periphery.
The absolute number of T cells gradually decreased according to the severity of the disease: A significant increase in the percentages of T cells (CD3+CD16-CD56-) was found in the mild group of patients with Covid-19 in comparison to the severe group and healthy control, as well (p<0.01, Table 2, Figure 4c). The absolute number of T cells was in the normal range in only the mild group of patients whereas this was found to be decreased in both moderate and severe groups in comparison to the mild group of patients and controls (p<0.01, Table 2).
No difference in the percentage of T helper cells but its number decreased in the moderate and severe COVID-19 groups:   The percentage of CD3+CD4+ T cells was not different among the groups (Table 2, Figure 4d). The absolute number of CD3+CD4+ T cells was in the normal range in the mild group. But, this was significantly reduced in moderate and severe disease courses in comparison to controls and also mild disease course (p<0.01; p<0.02 respectively, Table 2).
A normal number of T cytotoxic/suppressor cells with an increased percentage in the mild group decreased with the disease severity: The percentage of CD3+CD8+ cytotoxic/suppressor T cells was found to be elevated in the mild group in comparison to severe patients (p<0.03) and controls, as well (p<0.01; Table 2, Figure 4e). Thus, it was shown that the reason for the increase in the percentage of CD3+T cells in the mild group was due to the increase of CD8 + T cells. The absolute number of CD3+CD8+ T cells was in the normal range in the mild group. But, this was significantly decreased in the moderate and severe groups in comparison to the mild group and healthy controls (p<0.01, Table 2).
When the CD4 +CD8+ double-positive cells and the ratio of CD4/CD8 were examined among all the groups, no difference was detected.