How do invertebrates and animals without a specific immune system defend against invaders?

Animals with non specific defenses against invaders use physical barriers, protein defenses, cellular defenses, inflammation, and fever.
Physical barriers include skin and mucous membranes, and secretions separate the organism from its environment. Proteins that protect the body non specifically includes the complement protein found in the blood, when they bind to an invader they stimulate inflammation, phagocytosis, and destruction of the invaders membranes. The others include, inflammation, fever and cellular defenses. 

This post might have the info you need: