<P> Professional APCs specialize in presenting antigen to T cells . They are very efficient at internalizing antigens, either by phagocytosis (macrophages and dendritic cells) 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 . </P> <P> The main types of professional antigen - presenting cells are dendritic cells, macrophages and B cells . </P> <P> 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 . </P> <P> 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 . </P>

Where are antigen a and antigen b located