The adaptive and innate immune responses of vertebrates should not be considered separate independent systems but as interacting components of one system that provide complementary information to direct an inflammatory and immune response appropriately. In this chapter, we will examine two examples, the role of dendritic cell and T cell interactions and how complement functions with the humoral arm of the adaptive immune response. The innate immune response recognises molecules that are expressed on all pathogens, or damaged cells. It is commonly considered a rapid arm of the immune system, which responds swiftly in an antigen non-specific manner. This is in contrast to adaptive immunity, which is antigen specific and can take days to weeks to become effective due to the need for clonal expansion of antigen-specific T and B cells. The interaction of the dendritic cell with the naïve T cell is one of the central interactions of the immune system, as it provides the sole way in which the naïve T cell can be activated, thus initiating the T cell response. The outcome of this interaction is dependent on the phenotype of the dendritic cell, which is in itself a consequence of the interactions that the dendritic cell has had with molecules in its environment. For example, if the dendritic cell has been activated by molecules released from pathogens or damaged cells, then it will be activated and express co-stimulatory molecules that activate the T cell. It will also secrete cytokines that direct the differentiation of the T cell in a particular direction. This interaction can, therefore, be considered to have innate components, which provide context for the immune response, and adaptive components that provide specificity. The complement system contributes to defence against pathogens through interaction of molecules with components of cell surfaces that lead to activation of cascade in the presence of microbes. However, the complement system also contributes to humoral immunity through the classical pathway of activation, and through its role in B cell activation, affinity maturation and memory. This divide of the immune system into innate and adaptive responses can give the impression of two parallel immune systems, which sometimes interact. This is false. It is perhaps better to think about immune responses having both innate and adaptive components. Parts of the response recognise antigen in a specific manner, and other parts are activated by the context, the environment (presence of pathogens, damage, etc.). It is the coordination of these two components that are vital for effective immunological responses.