What are major barriers that protect us from disease?

Answer 1

The skin is the primary barrier that prevents infection. It forms a tight lattice of cells that do not allow (most) bacteria, viruses, or fungi through. Even water cannot pass easily. If the skin breaks, resident macrophages (immune cells) will try to stop or slow down the attackers. If skin is broken deep enough to rupture a vessel, the blood flowing out of the body will try to wash the bacteria out of the body.

Where there are openings to the inside of the body (mouth, nose, ears) there are mucous linings in place to prevent microbes from getting in. Cilia (microscopic fingers) move mucous in the lungs upwards constantly sending bacteria trapped in the mucous layer back up the way they came.

For those bacteria that try to come in through the stomach, a concentrated environment of hydrochloric acid found there will be toxic to most of them.

The brain has a special additional barrier called the blood brain barrier that is a junction even tighter than the skin to keep microbes out.

The immune system is an important second line of defense.