The immune system protects the body against harmful organisms, such as bacteria, viruses and parasites. The immune system includes the lymph nodes and an array of specialized cells that patrol the body organs and tissues and fight invading germs. Immune cells produce many types of specialized proteins that aid in the protective function of this system.
The immune system can be divided into two parts based on how specific their functions are - the innate immune system and the adaptive immune system.
For dogs, the innate immune system is their skin, stomach acid, mucous in the respiratory system and special chemicals in saliva.. This is the first line of defence, it is nonspecific and non adaptive.
The adaptive immune system defends the body against specific foreign invaders, designing different tactics for different invaders. The parts of the adaptive system communicate with each other and develop a memory of the various invaders they encounter.
If the invader is stopped by the innate system then no disease will occur. Otherwise the adaptive system is activated - if successful, the body will recover.