Introduction: Giardia lamblia and Plasmodium species are the pathogenic organisms for Giardiasis and Malaria, respectively, both human protozoan infections. Parasitic infections have continued to pose significant health challenges to children, especially in developing countries. Aim/Objective: This study investigated the immunohaematological effects of single and co-infections with Giardia lamblia and Plasmodium species among school-aged children (5–15 years) in Owerri West Local Government Area, Imo State, Nigeria. Methods: A total of 108 pupils were sampled from 20 randomly selected primary schools across rural, semi-urban, and urban settings. Blood and stool samples were collected for parasitological and immunohaematological analysis. Parasite identification was performed using direct microscopy: Stool samples for Giardia lamblia and thick/thin blood smears for Plasmodium spp. White blood cell (WBC) counts and differentials were measured using standard haematological techniques. Results: The total parasite load was 272.36×10?±1.32 parasites/µm, with Giardia and Plasmodium accounting for 76.2×10?±1.24 µm and 104.4×10?±0.88 µm, respectively. Co-infection was recorded at 91.76×10?±1.08 µm. The total WBC count in co-infected pupils (15.4×10?±0.03/L) was higher than in Giardia-only (6.95×10?±0.02/L), Plasmodium-only (7.3×10?±0.02/L), and control (7.05×10?±0.01/L) groups. Differential counts revealed marked elevations in total WBCs, neutrophils, eosinophils, basophils, monocytes, and lymphocytes in co-infection cases, with percentage values of 118.44, 34.38, 361.54, 566.67, 169.23, and 26.67%, respectively. Conclusion: The findings highlight the intensified immune response in co-infections and underscore the importance of inte grated surveillance for parasitic diseases.