'autoimmunity' originates from Greek and Latin elements: Greek 'autos' ('αὐτός') meaning 'self' and Latin 'immunis' meaning 'exempt, protected'.

'immunity' comes from Latin 'immunitas' (exemption from public duty) which passed into Medieval and then Modern English as 'immunity'; the prefix 'auto-' from Greek 'autos' was combined with 'immunity' in the 20th century to form the modern medical term 'autoimmunity', and 'autoimmunities' is simply the plural.

Initially 'immunity' meant 'exemption' (e.g., from public service); over time it shifted to mean biological resistance to disease. 'Autoimmunity' was coined to denote immune responses directed at the self and has retained that medical meaning; 'autoimmunities' refers to multiple such conditions or instances.