Steinernema carpocapsae (Weiser) has a symbiotic relationship with Xenorhabdus nematophila. The symbiotic bacteria delivered into the insect hemocoel by the nematode cause immunodepression of the target insect. In current study immune system of Spodoptera exigua Hübner against entomopathogenic nematode Steinernema carpocapsae, its symbiotic bacterium Xenorhabdus nematophila and some of involved factors in virulence were surveyed. Here, we evaluated total haemocytes count (THC), differential haemocytes count (DHC), activity of protease, phospholipase A2 (A2), phenoloxidase (PO) enzymes, expression of antimicrobial peptides (AMPs) attacin, cecropin, spodoptericin at specified intervals 0.5, 2, 4, 8, 12 and 16 hour post-injection (hpi). The infective juvenile larvae (IJs) were identified by S. exigua as a foreign agent but did not attach any haemocyte to nematode cuticle. Steinernema carpocapsae decreased haemocytes density by 12 hpi. The volume activity of PO and AMPs expression was suppressed by 4 and 8 hpi, respectively. The suppression of immune system was according to releasing time of symbiotic bacteria (4-8 hour after nematodes penetration into the haemocoel). Subsequently role of nematode and symbiotic bacteria in the suppression of immune system, three status including monoxenic nematode, axenic nematode and symbiotic bacteria were compared. The obtained results indicated the axenic nematode had weak defense response against immune system of S. exigua; while symbiotic bacteria was able to suppress the immune system and its ability was more than those against the nematode. In addition, the dead bacterium was able to suppress the immune system. According to the observations, the effect of the symbiotic bacteria on the immune system was explored more accurate. The outer membrane proteins (OMPs) of symbiotic bacteria were isolated and their effects on corresponding defense elements of S. exigua larvae was surveyed. In initial times post-injection OMPs increased haemocytes populations and activated defensive enzymes and AMPs expression. Then, haemocytes were destroyed and the enzymes activity and AMP expression suppressed. At last, pilin protein was injected into the haemocle of S. exigua. Total haemocytes count and granulocyte percentage were less than negative control by 0.5 to 16 hpi. But, pilin protein was not able to suppression of PO activity. Pilin protein induced expression of attacin, cecropin and spodoptericin and after a few hours decreased their expression. The study increased our knowledge about insect immune system against entomopathogenic nematode and its symbiotic bacteria. The gathered data highlighted the role of symbiotic bacteria during defense process of insect.