An effective adaptive immune response requires the presence of B cells, T cells, and APCs. For B cells to execute class-switch recombination and somatic hypermutation—prerequisites for production of high-affinity antibodies—they require help from activated T cells. These T cells, in turn, can be activated only by professional APCs such as dendritic cells. Dendritic cells sense the presence of pathogens through TLRs and other pattern-recognition receptors, such as the C-type lectins that can recognize polysaccharides and carbohydrate determinants. The interplay between components of the innate and adaptive immune systems is therefore a very important aspect of adaptive immunity. This layered, interwoven nature of innate and adaptive immunity both ensures a rapid early response of immediate protective value and primes the adaptive immune system for a specific response to any persisting pathogen. In this section, we describe how these various elements are activated and how the relevant cell types interact.