Prions are altered forms of proteins that are naturally produced in nerve cells and certain other cell types. Production of these normal proteins is illustrated from the nucleus at bottom right. When these "normal" proteins meet the rogue prions (purple) the normal protein changes shape to match the shape of the prions. It is this shape-flipping which is equivalent to the transmission of infection. Prions polymerise, finally appearing as purple fibrils in the picture. Prions could enter the brain along the axons of neurons, shown in the picture as purple spheres ascending the axon like elevators going up a shaft . Another route of entry could be the blood, probably in immune cells. A lymphocyte is shown exiting the capillary at bottom left where it could then contact the astrocyte. Astrocytes and other glial cells also support the production of prions.

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