The aim of this study is to assess the synthesis of kappa (κ) and lambda (λ) free light chains (FLCs) in the serum of patients with COVID-19. All the 120 serum samples were collected from patients with COVID-19 and from healthy controls (vaccinated and non-vaccinated against SARS-CoV-2). FLCs, IgG total, IgG4, IgG anti-Nucleocapsid (N), anti-spike S1 receptor binding domain (S-RBD) antibodies and IL-6 were measured according to the manufacturers’ instructions. The concentrations of anti-N IgG, IgG total, IgG4 and IL-6 were elevated in the COVID-19 group in comparison to the vaccinated and non-vaccinated controls. The levels of anti-S-RBD IgG and κFLC were increased in COVID-19 and healthy vaccinated patients when compared to non-vaccinated controls. λFLC concentration was higher in the COVID-19 group than in the non-vaccinated group. The κ:λ ratio was lower in both COVID-19 and non-vaccinated groups in comparison to vaccinated controls. κFLC correlated with all tested parameters (anti-S-RBD IgG, anti-N IgG, λFLC, κ:λ ratio, IgG total, IgG4 and IL-6) except CRP, whereas λFLC correlated with all examined parameters except IgG4. Elevated levels of FLCs in COVID-19 and healthy vaccinated against SARS-CoV-2 patients, as well as the correlation between free light chains with specific anti-SARS-CoV-2 antibodies and IL-6, reflect hyperactivation of the immune system after contact with coronavirus. Furthermore, it seems that serum levels of FLCs might be used as predictive markers of COVID-19. Our findings suggest that free light chains are involved in SARS-CoV-2 infection. However, understanding the exact mechanism requires further investigation.

Background: The aim of the study was to investigate the impact of COVID-19 on the pulmonary function tests (PFT) in COVID-19 convalescents six months after recovery. Additionally, the research question was whether PFT should be performed routinely in post-COVID-19 patients. Methods: A total of 39 patients with a history of COVID-19 6 months prior to the study were included in the study (Group I). Individuals were hospitalized or treated in the outpatients department. The control group (Group II) consisted of 39 healthy patients without a COVID-19 history. Each subject completed a questionnaire interview and underwent laboratory and pulmonary function examinations. Results: Six months after COVID-19 recovery, patients mainly complained about cough (46%, n = 18), shortness of breath (23%, n = 9), weakness (13%, n = 5), and memory/concentration disorders (8%, n = 3). In the group of patients complaining of persistent cough present 6 months after COVID-19, the following PFT parameters were decreased: FEV1, FVC, FRC, TLC, and DLCO (p < 0.05) in comparison with patients without this symptom. Conclusions: Persistent shortness of breath is not necessarily associated with pulmonary function impairment in patients 6 months after SARS-CoV-2 infection, and hence it requires appropriate differential diagnosis. Patients with a cough persisting 6 months after the acute phase of COVID-19 may benefit from PFT.