Fig. 1.  Two pathogenic components are essential to inducing adequate immune response, namely antigens (Ag) and pathogen-associated molecular patterns (PAMPs). After phagocytosis, antigens are processed in the lysosome and form a complex with human leukocyte antigen–DR (HLA-DR) on the surface of dendritic cells (DCs). This complex is recognized by T-cell receptors (TCR). Stimulation of PAMP-recognition receptors activates DCs and induces membrane expression of the costimulatory molecules CD80 and CD86. Activation of CD4+ T lymphocytes requires the membrane expression of two distinct signals: (1) HLA-peptide complexes, and (2) costimulatory CD80 and CD86. It is only when the same antigen-presenting cell expresses both signals that the T cell is activated.