Background: The immunomodulatory effect of helminths, such as Strongyloides stercoralis has indicated a possible protective effect in the increasing prevalence of allergic diseases.

Aim: The aim of the study was to assess the effects of S. stercoralis infection on the immune system of allergic patients. Methodology: A clinical analytical study was conducted in Basrah, Iraq, involving a total of 120 participants who were categorized as healthy control, allergic, and allergic and co-infected with S. stercoralis. ELISA and flow cytometry were used to determine immunological markers (IL-10, TGF-β, IL-4, IL-5) and regulatory T cells (Tregs).

Results: Co-infected individuals showed a considerable increase in CD4+CD25+FoxP3+ Tregs (7.80% ± 1.08% vs. 3.30% ± 0.66, P = 0.001) in comparison to allergic individuals alone. This was accompanied by a dramatic elevation in anti-inflammatory cytokines IL-10 (44.90 ± 7.90 vs 12.35 ± 4.10 pg/mL, P < 0.001) and TGF-β (342.59 ± 57.76 vs 177.70 ± 40.62 pg/mL, P < 0.001). Conversely, Th2-associated cytokines IL-4 and IL-5 showed significant suppression in the presence of S. stercoralis (16.91 ± 4.49 vs 24.38 ± 4.97 pg/mL and 15.27 ± 3.15 vs 23.18 ± 3.98 pg/mL, respectively, P < 0.01). The highest levels of total serum IgE occurred in the co-infected group (1213.92 ± 249.40 IU/mL) indicating a polyclonal response which is paradoxically accompanied by regulatory inhibition. There was a significant positive relationship between Treg expansion and the IL-10 levels (r = 0.78, P < 0.001).