The cell-mediated response is part of the immune response against infection of cells.

T cells are responsible fo this response. They differentiate under the influence of the thymus and respond to bacteria and viruses within infected body cells, also cancer cells and transplanted tissues.

A macrophage engulfs a bacteria and antigens are stripped and put on receptors which are presented on the surface of the macrophage. This antigen is presented to a virgin T cell. The T cell receptor must recognize botht the MHC-II protein and the specific antigen fragment to bind.

A CD4 protein also helps secure the relationship between the macrophage and the T cell.

The T cell becomes activated and divides into helper T cells, cytotoxic T cells.
The helper T cells help activate B cells in the humoral response.

The T cells also secrete interleukins, some of which cause a fever or help differentiation of other T cells.

The cytotoxic T cells are the effectors of the cell-mediated response. They search and destroy infected cells by secreteing chemicals that punches holes in the infected cell's membrane, causing lysis.

Dendritic cells also mediate a response because naive cytotoxic T lymphocytes do not display MHC-II molecules. An activated helper T-lymphocyte attaches it's TCR to the MHC-II of the dendritic call and this also involves the CD4 co-receptor. The CD4 co-receptor is necessary because even the license to kill needs a license to kill.

The Dendritic cell now has an activated helper t-lymphocyte attached. The MHC-I of the dendritic cell can now attach to the TCR of a Cytotoxic T lymphocyte along with a CD8 co-receptor.

The activated cytotoxic T lymphocyte seeks out virus infected cells and attaches its TCR to their MHC-1 along with the CD8 co-receptor. A release of cytotoxins lyses the infected cell and it is no more a threat.

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