foreign and self lipids to T cells and NK cells by using the CD1 family of proteins, which are structurally similar to the MHC class I family. Professional APCs Professional APCs specialize in presenting antigens to T cells. They are very efficient at internalizing antigens, either by phagocytosis (e.g. macrophages), or by receptor-mediated endocytosis (B cells), processing the antigen into peptide fragments and then displaying those peptides (bound to a class II MHC molecule) on their membrane. The T cell recognizes and interacts with the antigen-class II MHC molecule complex on the membrane of the antigen-presenting cell. An additional co-stimulatory signal is then produced by the antigen-presenting cell, leading to activation of the T cell. The expression of co-stimulatory molecules and MHC class II are defining features of professional APCs. All professional APCs also express MHC class I molecules as well. The main types of professional antigen-presenting cells are dendritic cells, macrophages and B cells. Dendritic cells (DCs) Dendritic cells have the broadest range of antigen presentation and are necessary for activation of naive T cells. DCs present antigen to both helper and cytotoxic T cells. They can also perform cross-presentation, a process by which they present exogenous antigen on MHC class I molecules to cytotoxic T cells. Cross-presentation allows for the activation of these T cells. Dendritic cells also play a role in peripheral tolerance, which contributes to prevention of auto-immune disease. Prior to encountering foreign antigen, dendritic cells express very low levels of MHC class II and co-stimulatory molecules on their cell surface. These immature dendritic cells are ineffective at presenting antigen to T helper cells. Once a dendritic cell's pattern-recognition receptors recognize a pathogen-associated molecular pattern, antigen is phagocytosed and the dendritic cell becomes activated, upregulating the expression of MHC class II molecules. It also upregulates several co-stimulatory molecules required for T cell activation, including CD40 and B7. The latter can interact with CD28 on the surface of a CD4+ T cell. The dendritic cell is then a fully mature professional APC. It moves from the tissue to lymph nodes, where it encounters and activates T cells. Macrophages Macrophages can be stimulated by T cell secretion of interferon. After this activation, macrophages are able to express MHC class II and co-stimulatory molecules, including the B7 complex and can present phagocytosed peptide fragments to helper T cells. Activation can assist pathogen-infected macrophages in clearing the infection. Deriving from