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An obvious indication of having a strong immune system is when a person rarely catches colds or other viruses. Bacterial and infections are also infrequent. If sickness does occur, it's usually milder and of shorter duration than for someone with a weaker immune system. There is a connection between immune response and mental status, so people with positive outlooks are often healthier than people who are prone to depression. People with strong immune systems don't suffer from excessive fatigue and are not easily stressed.
A healthy immune system works around the clock everyday of the year, most of the time doing its job completely unnoticed. It isn't until something goes wrong, like a catching a cold or having an infected wound that attention becomes drawn to it. Most components of the immune system aren't visible to the naked eye.
The skin is the largest organ of the body and is a part of the immune system. Part of the skin's immune function is obvious as it acts as a physical barrier to invading pathogens. Its other functions occur on a cellular level and aren't visible to the naked eye. If the skin becomes cut or torn, the opening allows germs, parasites, and other pathogens to enter the bloodstream.
Once a pathogen enters the bloodstream an immune response occurs, activating the innate, or non-specific, immune system to fight off the invading pathogens. The disease fighting cells of the innate immune system fight infectious invaders in a non-specific or generic way. If a more specified immune response is required, then the innate immune system triggers the adaptive immune system, which is only found in jawed vertebrates. The adaptive immune system has highly specialized disease fighting cells, which are able to adjust to newly encountered pathogens as well as remember specific pathogens in the event of a future attack.
Most of the time a strong immune system is able to fight off an infectious agent with little to no medical intervention. This doesn't mean, however, that needing medical assistance is a sign of a weakened immune system. Even a healthy immune system will require medical intervention if it encounters a particularly virulent infectious agent. Sometimes medical intervention is only needed to alleviate certain symptoms of an illness such as inflammation, which is, itself, a sign of a strong immune system at work. Inflammation is one of the earliest responses of an active immune system to an infectious or irritating agent.