淋巴细胞识别由主要组织相容性复合物（MHC）I或II类分子递呈的抗原肽片段。

T lymphocytes recognize antigenic peptide fragments presented by major histocompatibility complex(MHC) class I or class II molecules.

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在辅助T细胞激发针对病原的免疫反应之前，它必须感觉到位于抗原递呈细胞面表的入侵者分子片段。

Before a helper t cell can trigger a response to a pathogen or parasite, it has to "feel" these molecular bits of an invader displayed on the surface of an antigen-presenting cell (APC).

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第一信号为抗原与T细胞受体TCR的结合，第二信号为抗原递呈细胞上的协同刺激分子与T、B淋巴细胞上的受体结合。

The first signal is the binding of antigen with TCR on T cells. The second signal is the binding of co-stimulate on the APC with their receptors presenting on T and B lymphocytes.

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