If a pathogen gets beyond the natural barriers of the skin and lining of the gut, respiratory or urinary systems, and is not expelled mechanically it is confronted by the second line of defence, white blood cells. The defensive capabilities of white blood cells are still being unravelled and the names of certain cells are aptly descriptive. Your dog's natural defences depend upon not only macrophages (big eaters) which wander the body searching for bacteria, dirt, even cancer cells to engulf but also upon 'natural killer cells' and 'helper T cells'.
The natural defence system is awe-inspiring. Macrophages transport pathogens to other white blood cells that quite literally paint chemical labels on the pathogens labelling them for attack by larger white blood cells. To prevent the body's natural defences from attacking its own cells, every single cell in your pet's body carries photo identity imprinted on its surface technically called "major histocompatibility complex". Only when this superb defensive system fails or is overwhelmed do invaders get through. Even in these circumstances, other natural defences are available. White blood cells called 'memory T cells' act like a brain floating in your pet's circulation. When any of these cells meet the same pathogen in the future, they remember their past encounter and rapidly respond, stimulating production of antibodies to label the invader for immediate destruction. This is the basis for effective biological vaccination against infectious diseases.