The immune system has the daunting task of dealing with an amazing array of pathogens in our environment. Upon exposure to a pathogen, the innate arm of the immune system is engaged non-specifically. This is an important initial step in clearing the pathogen, however, adaptive immunity is required to generate a highly specific and efficient means of clearing the pathogen. The adaptive immune system is comprised of B and T lymphocytes that express receptors with remarkable diversity tailored to recognize aspects of particular pathogens, or “antigens”. B and T lymphocytes circulate in the blood and lymph and home to specialized lymphoid organs such as the spleen and lymph nodes. In these locations, inexperienced or “naïve” lymphocytes scan for the presence of antigens. During an infection, dendritic cells which act as sentinels in the peripheral tissues pick up pathogens in the form of antigenic determinants. These antigens are then presented to T lymphocytes within the lymphoid tissues. T lymphocytes of the appropriate specificity respond robustly to the antigen, and either kill the pathogen directly or secrete cytokine mediators that will encourage a B lympohocyte response. B lymphocytes provide humoral immunity by secreting antibodies specific for the pathogen. In the case of both B and T cells, as the immune response contracts, a small number of antigen-specific cells survive so that if re-exposure to the pathogen occurs, a more robust and rapid immune response can take place. This is termed immunological memory and it is what is conferred upon vaccination.