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Background: The ethiology behind lymphadenopathy syndrome (LAP) in children who frequently present with acute respiratory infections is not fully understood. Purpose – to study the characteristics of immune system in children who frequently present with lymphadenopathy syndrome.

Materials and methods: An immunological evaluation was conducted in four groups children aged 9–16 years. The first (main) group, (n=40), included those who frequently (6–8 times/year) presented with acute respiratory infections, recurrent bronchitis, and lymphadenopathy syndrome. The second group (comparison group (n=40)) those who also experienced acute respiratory infections and recurrent bronchitis 6–8 times/year but without, lymphadenopathy syndrome. The third group (comparison group (n=40)) presented with acute respiratory infections and acute bronchitis but did not have lymphadenopathy syndrome and were not categorized as frequently ill. The fourth (control (n=40)) group consisted of 30 healthy children of the same age. Local immunity was evaluated by measuring lysozyme, monomeric and dimeric IgA, IgG, and defensins concentration in saliva. Subpopulation composition of blood lymphocytes were studied by flow laser cytometry using specific monoclonal antibodies. The phagocytic activity of blood leukocytes was assessed using the thick drop method, based on their ability to ingest S. aureus.

Results: The obtained data indicate that acute respiratory infections in children of groups 1 and 2 occur against the background of reduced concentrations and activity of key humoral factors of local immunity: lysozyme, defensins, and sIgA. However, during the acute phase of the illness in these groups, a slight increase in IgG secretion in saliva was observed. After recovery, the levels of lysozyme, defensins, and sIgA did not return to normal, remaining significantly lower (p<0.05) compared to healthy subjects. In contrast, in group 3, the development of acute respiratory infections is accompanied by the activation of local protective factors. An increase in the concentration of monomeric and dimeric IgA, defensins, and lysozyme is observed in the secretion. After recovery, the levels of these factors in children of group 3 return to physiological norms.

Conclusions: Children who frequently suffer from acute respiratory infections with lymphadenopathy syndrome exhibit a combination of reduced overall immunoreactivity of the body and hyperactivity of certain lymphocyte subpopulations. For children with lymphadenopathy syndrome, both during the acute phase of the disease and after recovery, an increase in the levels of activated T-and B-lymphocytes with high cytokine-producing potential, as well as an increase in the polyclonal proliferative activity of lymphocytes, is characteristic.