Immunity has been suggested to be important in the pathogenesis of dementia. However, the contribution of innate versus adaptive immunity in the development of dementia is not clear. In this study, we aimed to investigate (1) the association between components of innate immunity (granulocytes and platelets) and adaptive immunity (lymphocytes) with risk of dementia and (2) the association between their derived ratios (granulocyte-to-lymphocyte ratio GLR, platelet-to-lymphocyte ratio PLR, and systemic immune-inflammation index SII), reflecting the balance between innate and adaptive immunity, with risk of dementia. Blood cell counts were measured repeatedly between 2002 and 2015 in dementia-free participants of the prospective population-based Rotterdam Study. Participants were followed-up for dementia until 1 January 2016. Joint models were used to determine the association between granulocyte, platelets, and lymphocyte counts, and their derived ratios with risk of dementia. Of the 8313 participants (mean standard deviation age 61.1 7.4 years, 56.9% women), 664 (8.0%) developed dementia during a median follow-up of 8.6 years. Doubling of granulocyte and platelet counts tended to be associated with an increased risk of dementia (HR 95%CI 1.22 0.89-1.67 and 1.45 1.07-1.95, respectively). Doubling of the derived ratios GLR, PLR, and SII were all associated with an increased dementia risk (HR 95%CI 1.26 1.03-1.53, 1.27 1.05-1.53, and 1.15 0.98-1.34, respectively). GLR, PLR, and SII are associated with an increased risk of dementia in the general population. This supports the role of an imbalance in the immune system towards innate immunity in the pathogenesis of dementia.