Immune System and Life

As the clarity of vision fades or the acuity of hearing diminishes, so does the efficiency of the immune system.

It still works well most of the time, but not as it once did. Failures tend to happen more frequently.

For example, the immune system may fail to detect and destroy a bacterium or virus. This leads to a greater incidence of infections, such as bacterial skin infections, in older dogs.

The immune system may fail to detect a cell that has been duplicated incorrectly by a tissue or organ.

This is why tumors - both benign ones such as lipomas and malignant ones such as osteosarcomas {bone cancers) — are much more common in older dogs than in youngsters.

As a dog ages its immune system sometimes underreacts to threats, but it may also overreact.

For example, it may erroneously think that part of the body is ''foreign," and so attack and destroy it. This process is the most common cause of thyroid gland failure in older dogs.