|The harlequin ladybird, Harmonia axyridis, is considered to be one of the world’s most important invasive insects. It has been spread over substantial part of Europe and North America in last decades and its invasion is potentially detrimental for native ladybird species. The powerful immune system makes harlequin ladybird highly resistant to diseases that affect other ladybird species and is probably one of the main reasons of its successful invasiveness. In this study, we examined the basic immune parameters including concentration of haemocytes and antimicrobial activity in haemolymph during ontogenetic development (from the 2nd larval instar to the 32 days old adults). The concentration of haemocytes in haemolymph of H. axyridis does not change significantly during larval stages and is quite low until the prepupal stage (about 5500 haemocytes per 1 ul of haemolymph). After the metamorphosis the concentration of haemocytes rapidly increases during first eight days of adulthood and then it remains constant (around 30000 haemocytes per 1 ul of haemolymph). Similarly to the haemocyte level, the protein content in haemolymph was significantly influenced by the metamorphosis which caused the temporal decrease in protein concentration between prepupa and newly hatched adults. Subsequently, the protein concentration gradually increased within first 16 days of adulthood. The haemolymph of H. axyridis exhibited very high antimicrobial activity against Escherichia coli which increased from larval stages till adults. Surprisingly, we did not detect any significant differences in measured parameters between sexes of H. axyridis adults.