<unk> -restricted antigen recognition, or<unk> , refers to the fact that a T cell can interact with a self-major histocompatibility complex molecule and a foreign peptide bound to it, but will only respond to the antigen when it is bound to a particular<unk>  molecule. When foreign proteins enter a cell, they are broken into smaller pieces called peptides. These peptides, also known as antigens, can derive from pathogens such as viruses or intracellular bacteria. Foreign peptides are brought to the surface of the cell and presented to T cells by proteins called the major histocompatibility complex <unk> . During T cell development, T cells go through a selection process in the thymus to ensure that the T cell receptor (TCR) will not recognize<unk>  molecule presenting self-antigens, or rather it has a moderate affinity. High affinity means it will be autoreactive, but no affinity means it will not bind strongly enough to the<unk> . The selection process results in developed T cells with specific TCRs that might only respond to certain<unk>  molecules but not others. The fact that the TCR will recognize only some<unk>  molecules but not others contributes to<unk> ". The biological reason of<unk>  is to prevent supernumerary wandering lymphocytes generation, hence energy saving and economy of cell-building materials. T-cells are a type of lymphocyte that is significant in the immune system to activate other immune cells. T-cells will recognize foreign peptides through T-cell receptors (TCRs) on the surface of the T cells, and then perform different roles depending on the type of T cell they are in order to defend the host from the foreign peptide, which may have come from pathogens like bacteria, viruses or parasites. Enforcing the<unk>  that T cells are activated by peptide antigens only when the antigens are bound to self-MHC molecules,<unk>  adds another dimension to the specificity of T cell receptors so that an antigen is recognized only as peptide-MHC complexes.<unk>  in T cells occurs during their development in the thymus, specifically positive selection. Only the thymocytes (developing T cells in the thymus) that are capable of binding, with an appropriate affinity, with the<unk>  molecules can receive a survival signal and go on to the next level of selection.<unk>  is significant for T cells to function properly when it leaves the thymus because it allows T cell receptors to bind to<unk>  and detect cells that are infected by intracellular pathogens, viral proteins and bearing genetic defects. Two models explaining how<unk>  arose are the germline model and the selection model. The germline model suggests that<unk>  is a result of evolutionary pressure favoring T cell receptors that are capable of binding to<unk> . The selection model suggests that not all T cell receptors show<unk> , however only the T cell receptors with<unk>  are expressed after thymus selection. In fact, both hypotheses are reflected in the determination of TCR<unk> , such that both germline-encoded interactions between TCR and<unk>  and co-receptor interactions with CD4 or CD8 to signal T cell maturation occur during selection.
MHC restriction
Sex selection
Fragment antigen-binding
Affinity maturation
Functional specialization 
Alloantigen recognition
Clonal selection
HIV integration