Alder-Reilly's anomaly is characterized by aberrant granulation of almost all neutrophilic granulocytes. Initially, this anomaly looks much like toxic granulation from infection or chemotherapy. However, it is characterized by the presence of inhomogeneously scattered clusters of very coarse granules. Aberrant or increased granulation may additionally occur in eosinophils, basophils, lymphocytes and monocytes.

This abnormality is not a result of inflammation, but is hereditary. There is a strong association with certain lysosomal storage disorders (mucopolysaccharidoses) and Tay-Sachs disease.