Two lymphocytes are present in this image of blood from a cat. The cell on the left is a granular lymphocyte – a small grouping of fine red granules is seen in the indentation of the nucleus. These are usually cytotoxic (CD3+CD8+) T cells or natural killer cells and the granules contain cytolytic enzymes, such as perforin and granzyme B, which facilitate cytotoxicity. Low numbers of these cells (comprising between 5-10% of total lymphocytes, depending on the species) can be seen in the blood from healthy animals. In this case, the proportion of granular lymphocytes was increased, suggesting an antigenic or reactive response. A reactive lymphocyte (deeper blue cytoplasm than normal) is present on the right, indicating antigenic stimulation.
