Measles
Pathophysiology

Once the measles virus gets onto the mucosa, it infects the epithelial cells in the trachea or bronchi. Measles virus uses a protein on its surface called hemagglutinin (H protein), to bind to a target receptor on the host cell, which could be CD46, which is expressed on all nucleated human cells, CD150, aka signaling lymphocyte activation molecule or SLAM, which is found on immune cells like B or T cells, and antigen-presenting cells, or nectin-4, a cellular adhesion molecule. Once bound, the fusion, or F protein helps the virus fuse with the membrane and ultimately get inside the cell.

As the virus is a single-stranded negative-sense RNA virus, it includes the enzyme RNA-dependent RNA polymerase (RdRp) which is used to transcribe its genome into a positive-sense mRNA strand.

After entering a cell, it is ready to be translated into viral proteins, wrapped in the cell's lipid envelope, and sent out of the cell as a newly made virus. Within days, the measles virus spreads through local tissue and is picked up by dendritic cells and alveolar macrophages, and carried from that local tissue in the lungs to the local lymph nodes. From there it continues to spread, eventually getting into the blood and spreading to more lung tissue, as well as other organs like the intestines and the brain. Functional impairment of the infected dendritic cells by the measles virus is thought to contribute to measles-induced immunosuppression.
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