Antigen Presentation and Lymphocyte Activation
The peripheral lymphoid organs are the primary meeting place between cells of the innate immune system (APCs) and cells of the adaptive immune system (T-cells and B-cells). Upon interaction with APCs, pathogen-specific T-cells and B-cells will be activated, provided that they acquire the appropriate signals from the APCs. Besides require co-stimulatory signals via interaction of accessory and co-stimulatory molecules between lymphocytes and APCs. This cell-cell interaction is essential for proper stimulation of lymphocytes and without those accessory signals, antigen-specific T-cells may become anergic. Lymphocytes receiving the appropriate signals for activation will clonally expand and generate multiple progenitors all recog-nizing the same antigen. Clonalexpansion is a typical feature of the adaptive immune system, which will be discussed in more detail below.