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What are T cells and how do they help immunity?

T cells might play a crucial role in COVID-19 vaccines efficacy against variants.

T lymphocyte cells attacking a cancer cell, computer illustration. T lymphocytes are a type of white blood cell that recognise a specific site (antigen) on the surface of cancer cells or pathogens and bind to it. Some T lymphocytes then signal for other immune system cells to eliminate the cell. The genetic changes that cause a cell to become cancerous lead to the presentation of tumour antigens on the cell's surface.
T lymphocyte cells (white) attacking a cancer cell (red). Credit: KATERYNA KON/SCIENCE PHOTO LIBRARY/Getty Images.