When a blood smear is made, the cells undergo a certain degree of mechanical stress, which can cause fragile leukocytes to lyse. Most often these are lymphocytes. In other cells, certain structures can often still be recognized (such as the granules), whereas in a smeared lymphocyte, usually only a smeared nucleus remains.

Fragile leukocytes originate from strong activation, such as in response to infection, or due to improper or hasty production in malignancies. In particular, acute viral infections and chronic lymphocytic leukemias are typically characterized by large numbers of smudged lymphocytes. Only in the latter case are they also referred to as "basket cells" or "gumprecht shadows".