There are two major components to immunity. The first component called "recognition" occurs when a foreign agent invades the body for the first time. The immune system recognizes the agent as foreign and within a short time, a series of reactions begin which eventually destroy the invader. Because it takes time for the immune system to launch its defense, first-time invaders usually will produce symptoms of illness, and severity will depend on the extent of exposure and invasiveness of the enemy agent. However, once the first-time invaders are destroyed, if that particular enemy agent attempts to invade at a later time, the immune response will occur much more rapidly and the body will experience few or no symptoms before the agent is destroyed.

The second component of the immune system is called "discrimination". The immune system must be able to differentiate the normal tissues and fluids that make up the canine body from the invading agent. To facilitate this, the cells of the dog's own body have a unique set of molecules on their surface which allows the dog's immune system to recognize them as its own cells. However, the molecules on the surface of an invading agent, called antigens, will be different and will, therefore, allow the immune system to recognize the agent as foreign and initiate a defense.

Therefore, for normal immune system function, both of these components, recognition and discrimination, are vital for the survival of the dog.