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Activation of T Lymphocytes
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Activation and effector phases of T cell-mediated adaptive immune responses
are triggered by antigen recognition by T lymphocytes
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Naive T lymphocytes home to secondary lymphoid organs, where they may
encounter antigens presented by mature dendritic cells on class I or class II
MHC molecules and thus become activated

Antigen-stimulated T cells that have received both "signal one" through the
antigen receptor and "second signals" via co-stimulatory receptors may be
induced to secrete cytokines and to express cytokine receptors (like IL-2)

T cell responses decline after the antigen is eliminated by effector cells

The proliferation of T lymphocytes and their differentiation into effector and
memory cells require antigen recognition, costimulation, and cytokines

Antigen is always the necessary first signal for the activation of lymphocytes,
ensuring that the resultant immune response is specific for the antigen

Activation of naive T cells requires recognition of antigen presented by
dendritic cells

Elimination of antigen leads to contraction of the T cell response, and this
decline is responsible for maintaining homeostasis in the immune system

Costimulation and growth factors like IL-2 stimulate expression of the antiapoptotic proteins Bcl-2 and Bcl-XL in the activated lymphocytes, and these
proteins keep cells viable

The inhibitory receptors CTLA-4 and PD-1, apoptosis induced by death
receptors of the TNF receptor superfamily (such as TNFRI and Fas), and
regulatory T cells