Is the only effective way invertebrates fight disease. phagocytosis; excretion; lysozyme

Excretion is the only effective way invertebrates fight disease. Unlike vertebrates who have immune systems that work daily to fight off bacteria or other harmful organisms, invertebrates only have the capacity to excrete and cleanse their body of harmful materials.

Phagocytosis

Unlike humans and other vertebrates that have complex immune systems, these invertebrates don't. Further, when a pathogen breaches the skin and  enters the body, it meets the first line of (systemic ) defense inside the body, namely white blood cells called neutrophils. Neutrophils are always in circulation in the blood and when they meet any foreign body, they attack it by engulfing it (phagocytosis)  and kill it.