Lewy bodies are abnormal protein aggregations that develop inside nerve cells in Parkinson's disease, Lewy body dementia, and some other neurological disorders.
They are identified under the microscope when histology is performed on the brain (Figure 1) and their primary structural component is alpha-synuclein.
Figure 1. Histology sections of different regions of the substantia nigra in a Parkinson's patient showing Lewy bodies (pointed with an arrow). Image adapted from Suraj Rajan under Creative Commons license.