Figure 2: The normal immune response following influenza vaccination. Administration of vaccine antigens induces the activation of the innate immune responses at the site of injection. The antigen is taken up by antigen-presenting cells (1), such as macrophages and dendritic cells (DCs). The local innate immune response facilitates maturation of DCs, which present stable major histocompatibility complex/peptide complexes (2). Mature DCs migrate into lymph nodes (3), where they induce activation and clonal expansion of naive CD4+ (4) and CD8+ (5) T cells. The activation and differentiation of naive B cells is induced by antigen and CD4+ T helper cells (6). Naive B cells differentiate into memory B cells and antibody-secreting B cells (7). Long-term immunity is assured by memory B and T cells in the blood and lymph nodes, as well as by long-lived plasma cells and memory T cells in the bone marrow (adapted from [17]).