As soon as blood leaves the human body, stress to the host´s protective cellular and protein cascades begin. My thesis focuses on the immune system, specifically the complement system and platelets, primarily known for their hemostatic properties but also play a role in immune responses. The core of this thesis is the interaction between the complement system and platelets in platelet concentrates destined for transfusion. Storing blood products, particularly platelet concentrates, presents challenges, including the development of platelet storage lesions. These lesions involve a series of biochemical, structural, and functional changes from when platelets are collected to when they are transfused, potentially leading to impaired platelet function and adverse transfusion reactions. For inventory management, the oldest platelet concentrates are typically used first for transfusion. Understanding the interplay between the complement system and platelets during storage is crucial for improving the quality of platelet concentrates for selecting optimal concentrates based on the indication. This thesis includes several exploratory studies: one examining the complement system and platelet function over storage time, another investigating the impact of complement inhibition with the aim to reduce platelet storage lesions, and a third exploring the ability of platelets to release mediators that can modulate an immune response when supplemented to thrombocytopenic blood. Additionally, one study examines the impact of blood collection tube composition on complement functional and activation analysis. I found that increased storage time was associated with increased complement activation, increased platelet activation and attenuated platelet function. However, there was no causal relationship between complement and platelet activation since complement inhibition did not alter platelet activation or function. Further, I found that platelets release mediators that could modulate an inflammatory reaction, but the storage time had only minor effect on the immunomodulatory effect. Last, I found that the composition of blood collection tubes significantly affected complement activation. While these findings may not immediately benefit patients, they provide new insights into platelet concentrates, highlighting the role of the complement system. Further research is needed to to understand the interaction between these components fully.