It is well established that allergic asthma is T cell-driven disease where CD4+ T cells of Th2 phenotype play a critical role in disease initiation and maintenance. There are several critical steps in the induction of Th2 type immune response to the allergen. The first critical step is the antigen processing and presentation of allergen-derived peptides in the context of specific major histocompatibility Class II (MHCII) molecules by antigen-presenting cells (APC). Recognition of this complex

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... T cell receptor (TCR) and interaction of costimulatory ligands with corresponding receptors represents the second step in T cell activation. As the third part of optimal T cell differentiation, proliferation, and expansion, several cytokines, integrins, and chemokines get involved in the fine-tuning of DC-T cell interaction and activation. Multiple recent evidences point to the selected members of B7 and semaphorin families as important checkpoints providing a fine-tuning regulation of immune response. In this book chapter, we discuss the properties of costimulatory molecules and address their roles in allergic asthma.