Organ in the immune system (6)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
spleen
(Other definitions for spleen that I've seen before include "Body organ; spite" , "Abdominal organ - bad temper" , "Bodily organ associated with bad temper" , "Acrimony" , "Peevishness, from bodily organ" .)