Understand at a glance: Should we get the “fourth shot” of the new coronavirus vaccine?

Vaccination with the new coronavirus vaccine can effectively reduce the risk of severe illness and death, reduce the probability of secondary infection, and reduce the probability of hospitalization after infection. Recently, the second dose of the new coronavirus vaccine, commonly known as the "fourth shot", has begun to be administered in various places. Who needs to be vaccinated? What are the benefits of vaccination? Can children and pregnant women be vaccinated? Regarding the hot questions about the "fourth shot" of the new coronavirus vaccine, NetEase Health summarizes it for you in one picture!

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①Three lines of human immune defense:

First line of defense: skin and mucous membranes, etc.

The second line of defense: bactericidal substances and phagocytes in body fluids.

The third line of defense: immune organs and immune cells.

**The first two lines of defense are nonspecific immunity, **also known as innate immunity or inherent immunity, which refers to the normal physiological defense function that the body possesses by nature, and can make corresponding immune responses to the invasion of various pathogenic microorganisms and foreign bodies.

**The third line of defense is specific immunity,** which is specifically targeted at the recognition and killing of a certain pathogen (antigen).

② Antigen and Antibody

**Antigen: ** refers to a substance that can cause the human body to produce a specific immune response. Its essence is the protein on the surface of pathogens such as viruses and bacteria. Vaccines are also a type of antigen.

**Antibody: ** refers to the protein with protective effect produced by the body due to the stimulation of antigen.

③Immune memory and cellular immunity

**Immune memory: **When a virus invades the human body for the first time, immune cells will identify, activate, proliferate, and differentiate the virus. When the same virus enters the human body for the second time, the immune system will quickly initiate an immune response again. This phenomenon shows that the immune system has memory, that is, immune memory.

The "combat troops" of the human body's third line of defense are mainly lymphocytes, and their "combat strategies" are divided into humoral immunity and cellular immunity.

Humoral immunity: Fighting against pathogens in extracellular fluid , mainly involving B lymphocytes.

Cellular immunity: Fights against pathogens that have already invaded cells , mainly involving T lymphocytes.