Helper T cells are thought to be the most important cells in adaptive immunity, as they are required for most adaptive immune responses.
They not only help activate antibody-secreting B cells and macrophages to destroy ingested bacteria, but also help activate cytotoxic T cells to destroy infected target cells.
As has been clearly demonstrated in AIDS patients, without helper T cells we cannot protect ourselves against many normally harmless bacteria.
However, helper T cells themselves can only function when they are activated to become effector cells. They are activated on the surface of antigen-presenting cells, which mature during the innate immune response to infection.
Innate responses also regulate the type of effector cells that helper T cells develop and thus determine the nature of the activated adaptive immune response.
Unmatched T cells require at least two signals to be activated. Both are provided by antigen-presenting cells, usually dendritic cells: signal 1 is provided by the MHC peptide complex that binds to the T-cell receptor, while signal 2 is mainly provided by CD28-binding B7-stimulating proteins on the surface of T cells.
If the T cell receives only signal 1, it is usually suppressed or inactivated.
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