1st line of defense: Physical barriers including skin, saliva, nose hair, etc. Mainly to prevent bacteria & viruses from invading the body

2nd line of defense: Phagocytic cells, antibacterial proteins, inflammatory response. The body produces immune and anti-epidemic response to achieve defense

3rd line of defense: T cell, B cell, NK cell(nature killer) one-to-one protection, work on specific pathogens, in cell-mediated immunity

Do you know??
"Nose hair" is also the first line of defense for breathing
The nose is the breathing channel. The human body inhales nearly 10,000 liters of air every day. Everyone knows that the nose hair can block dust. Obviously, the nose hair is the first pass of the respiratory tract. When people breathe, the air passes through the nasal cavity, and most of the bacteria and dust in the air are blocked by the nasal hairs. That is to say, the nasal hairs play a certain role in filtering the inhaled air; because of the presence of nose hairs, the cold air will also become warm when it enters the nasal cavity, and dry or cold air will become warm and moist when filtered through the nasal cavity. In addition, nasal hair can effectively help the nasal cavity excrete secretions.

The "lung" is the body's first line of defense
Every breath we breathe is mixed with dust, bacteria, and even viruses! ! The "lung" is the first human organ to help us filter out bacteria or viruses that are harmful to our body! Once bacteria or viruses invade, it will break your respiratory tract, mild cases like a common cold, severe cases will develop into pneumonia.