During an infection, different immune cells must mount a coordinated defense against foreign invaders. This requires communication. Immune cells talk to and influence one another either by direct cell-cell interactions or by secreting factors that bind to and activate each other. Cell-cell interactions occur via receptors that are unique to certain immune cells. Secreted factors that activate other immune cells include molecules called cytokines and interferons.
T Cell Receptors and MHC Receptors
The T cell receptor (TCR) is expressed on T lymphocytes (T cells), which are vital to the body's immune response. The TCR is what a T cell uses to directly communicate with a cell that has been infected by a foreign invader. The infected cell presents at its surface a piece of the invader. It presents this piece via a receptor called major histocompatibility complex I (MHCI). A special type of T cell -- called the helper T cell -- and the infected cell then “hold hands” by connecting TCR to MHCI, with the foreign particle sandwiched in between.
CD4 and CD8 Receptors
T cells come in different varieties. One way of categorizing them is by the presence of receptor proteins called CD4 or CD8 on their surface. T cells that have CD4 are called helper T cells -- these activate other immune cells. T cells that have CD8 are called cytotoxic T cells -- these kill infected cells. Two types of MHC receptors, MHCI and MHCII, present foreign particles for T cells to recognize. T cells that have CD4 bind to cells that have MHCI, while T cells that have CD8 bind to cells that have MHCII.
Cytokines & Chemokines
Immune cells can communicate with each other by directly binding to receptors on each other’s surfaces. They can release proteins called cytokines and chemokines, which flow away and bind to the surface of a cell that is nearby or far away. Cytokines are small proteins that are released from an immune cell and can activate the cell that released it, a neighboring cell or a cell that is far away. Chemokines are small proteins that attract immune cells. Chemokines serve as the “come hither” perfume that some immune cells release in order to attract more immune cells to a certain location.
Another factor secreted by immune cells as a form of communication consists of molecules called interferons (IFN). The three classes of interferons are alpha, beta and gamma. IFN-alpha is secreted by immune cells that have been infected by a virus. IFN-beta is secreted by a nonimmune cell that has been infected by a virus. IFN-gamma is secreted by T cells that have been activated for battle against invaders. The common purpose of all three IFNs is to increase the amount of MHCI receptors in cells, so that T cells, which bind to MHCI receptors, are more likely to find cells that have been infected.
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