The most recent technology to be proposed is the peptide-MHC microarray or artificial antigen-presenting chip [9, 11, 99] ; in this case, recombinant peptide-MHC complexes and costimulatory molecules are immobilized on a surface, and populations of T cells are incubated with the microarrays, whose spots effectively act as artificial antigen-presenting cells [100] containing a defined MHC-restricted peptide. Different methods have been proposed for detecting T cells expressing receptors with affinity for specific peptide-MHC complexes on the microarray; these can include simple inspection of T cell clusters bound to a spot [9] or identification of activated cells secreting specific cytokines with cytokine-specific capture antibodies [11, 99] . Peptide-MHC microarrays correspond to high-throughput parallelized ELISPOT assays [19] , particularly when low enough densities of cells are used, in which case a direct counting of activated cells is possible [11] . Quantitation can involve cell counts alone, detected cytokine intensities alone, or a combination of both, as in [99] , which used a cell-count score adjusted by an intensity score. The benefit of using peptide-MHC microarrays is that it can map MHC-restricted T cell epitopes, which are involved in several helper and regulatory functions of the immune system, and can be used in conjunction with peptide-based B cell epitope microarrays to study the adaptive system as whole. Figure 2 illustrates the functioning of peptide-MHC microarrays. In Figure 2a , a peptide-MHC microarray is depicted, with an inset showing the probe molecules that are deposited on a microarray spot. Figure 2b depicts T cells that bind to, and are activated by, specific peptide-MHC complexes, with the help of co-stimulatory antibodies; these T cells secrete cytokines that are captured by specific detection antibodies. Finally, as depicted in Figure 2c , the T cells and excess cytokine are washed away, and the bound cytokine is revealed by fluorescent antibody (other methods can be employed to reveal the cytokine [11] ). Therefore, a peptide-MHC spot is designed taking into account two elements: a peptide-MHC complex, and the detection antibody specific to the particular cytokine one wants to measure. A third element can be the kind of T cell population (e.g., T helper or CTL) that is used as a target (i.e., incubated with the microarray). The choice among these three different elements will lead to a vast number of immunomic responses that can be measured.