What type of immune response is primarily affected in X-linked lymphoproliferative syndrome?

In X-linked lymphoproliferative syndrome (XLP), the primary defect is related to cell-mediated immunity, specifically affecting the ability of T cells to respond effectively to certain viral infections, particularly Epstein-Barr virus (EBV). Patients with XLP often display a failure of T cell activation and proliferation due to mutations in the SH2D1A gene, which plays a crucial role in signaling pathways necessary for T cell function.

While humoral immunity, mediated by B cells and the production of antibodies, also plays a role in the immune response, the key feature of XLP is the impaired function of T cells, which are central to cell-mediated immunity. This results in an inability to control viral infections effectively and can lead to lymphoproliferation and increased risk of lymphomas.

Innate immunity refers to the body's first line of defense against pathogens and is not specifically compromised in XLP. Similarly, complement-mediated immunity, which involves proteins that enhance the ability of antibodies and phagocytic cells to clear pathogens, is not primarily impacted in this condition. Hence, the core issue in XLP is related to cell-mediated immunity, making it the correct answer to the question.