Most of us are more familiar with the musculoskeletal system than we care to be. That low back pain or twisted ankle was a rude introduction. And that hacking cough brought us up close and personal with the respiratory system.
But there’s another system that works silently in the background. It’s on constant guard looking for intruders — something that should not be there. The immune system can distinguish between normal, healthy cells and unhealthy cells. When it detects an outsider — bacteria or a virus, for example — it readies forces to eradicate it.
What’s even better, it can often remember. If the virus tries to enter again, its entry is blocked.
There are two types of immunity. Natural immunity occurs when a person contracts a particular disease — measles, for example. Many people born before 1957 were infected with the disease. Most are now permanently protected
Click to read the full article @The Bay State Banner