Lewy bodies are the inclusion bodies – abnormal aggregations of protein – that develop inside nerve cells affected by Parkinson's disease (PD), the Lewy body dementias (Parkinson's disease dementia and dementia with Lewy bodies), and some other disorders. They are also seen in cases of multiple system atrophy, particularly the parkinsonian variant (MSA-P).They are identified in tissue examined under the microscope. Lewy bodies appear as spherical masses in the cytoplasm that displace other cell components. Lewy bodies may be found in the midbrain (within the substantia nigra) or within the cortex. A classical Lewy body is an eosinophilic cytoplasmic inclusion consisting of a dense core surrounded by a halo of 10-nm-wide radiating fibrils, the primary structural component of which is alpha-synuclein. Cortical Lewy bodies are also composed of alpha-synuclein fibrils, but are less defined and lack halos. In histopathology, cortical Lewy bodies are a distinguishing feature for dementia with Lewy bodies, but may occasionally be seen in ballooned neurons characteristic of behavioural variant frontotemporal dementia and corticobasal degeneration, as well as in patients with other tauopathies.