To you who are fighting the virus!

This article was first published in 2020

Recently, many people's immune systems

Fighting the virus

In order to facilitate understanding of the "battle situation", this article is reposted

I wish you all a speedy recovery.

Our life

Destined to fight the virus

They are huge in number

Across land and sea

And the air we breathe

Some scientists estimate

There are more than 1,000 viral particles in the ocean alone.

10000000000000000000000000000000

That is 1×10³¹

(The number of viruses is estimated by microbiologist Lita Proctor; the large number of purple particles in the picture below are Middle East Respiratory Syndrome Coronavirus, attached to green cells, imaged by scanning electron microscopy and later colored, the picture comes from @NIAID)

▼

They act cunningly

Can infect all living things

Can break through species barriers

Between humans and animals

Cross-species transmission

(Civet cat, some studies believe it is one of the intermediate hosts of SARS coronavirus, photographer @邹涛)

▼

They are cruel

Can put people to death

Arouse high vigilance in human society

Even severe panic

Panic people

Don't know how the virus will treat them

Not knowing your own body

What will change

This article will reveal to you

All this

Round 1

Virus Attack

A virus is a parasite

They have no cellular structure

Must invade organisms such as humans

Proliferate with the help of cells of an organism

therefore

Invasion

Invasion by any means

The virus's only target

(After the influenza virus has completed its proliferation in the cell, it is budding and released from the cell surface. The yellow-green color is the virus. Transmission electron microscopy imaging. The picture comes from @NIAID)

▼

They wander around

Find the flaws in your body

Including skin and mucous membranes

Five major invasion directions

First of all, your

skin

Skin covers our body

Extensive contact with the outside world

However, the outermost layer of the skin

The stratum corneum is composed of dead cells

Extremely difficult to break through

Almost no viruses

Can invade from here

(Schematic diagram of the epidermal structure of the palms and soles of the feet. The skin in other parts of the body does not have a transparent layer. Map by @赵榜/Planetary Research Institute)

▼

but

When you are bitten by mosquitoes, scratched by cats and dogs

Tattoos, abrasions, skin ulcers

and injections with unclean needles

Will cause loopholes in the epidermis' defense line

Dengue virus and Zika virus transmitted by mosquitoes

and rabies virus spread by animals such as cats and dogs

Multiple viruses

They will take advantage of the opportunity

so

Please take care of your skin

Don't destroy the Great Wall

(Illustration of mosquito bites, drawn by @Zheng Borong/Planetary Research Institute)

▼

Since the skin cannot penetrate

Then the same exposed

Eye

It became the second invasion direction

However

The eyelid that covers the eyeball (conjunctiva)

Every few seconds

Will scan the eyeball once

Use its secretions to clean the eyeball and remove foreign particles

Less chance for the virus to infect the eyes

But some polluted environments

Still possible to cause harm

For example, an unclean swimming pool

Or touching your eyes with infected hands

Or the virus in the air suspension contacts the conjunctiva

(This is also why medical staff need to wear goggles or even masks. The picture below was taken in Huangshi City, Hubei Province, by photographer @何戈)

▼

The next third invasion direction

Urogenital tract

It's not that easy

Taking the female vagina as an example

Continuous shedding of vaginal epithelial cells

Not conducive to virus attachment

And the vagina is usually protected by mucus

Lactic acid bacteria that are beneficial to the human body

I will choose to live here

Creates a low pH acidic environment

Many viruses are sensitive to acidic environments

Can only retreat

(Lactobacillus acidophilus, a type of vaginal flora, image from @Wikimedia commons)

▼

What breaks through the vaginal defense line

Human sexuality

During sexual intercourse

Destruction or abrasion of the vaginal epithelium

The virus has the opportunity to invade the inner layer

Notorious

Human immunodeficiency virus

(HIV, also known as AIDS virus)

Even without wear and tear

Squeeze right through the epithelial cells

A moment of joy

You're in trouble.

(HIV targets lymphocytes in the human immune system. The yellow sphere in the picture below is HIV on the surface of lymphocytes, imaged by a scanning electron microscope. Image source: @VCG)

▼

The fourth invasion direction is

Digestive tract

The environment here is very good for viruses

Still “bad”

They first encounter in the mouth

Saliva can kill many viruses

When it reaches the stomach

Then you will encounter stomach acid and digestive enzymes

It's hard to reach the intestine

The bile killed them almost without leaving any

There are many beneficial bacteria in the intestines

It also increases the difficulty of the virus's conquest of cities and territories.

It can be said to be layered defense

(Diagram of the digestive tract "defense line", drawn by @Zheng Borong/Planetary Research Institute)

▼

Viruses that can survive under these conditions

Often requires some special skills

Rotavirus, which can cause gastroenteritis

Has a three-layer protein protective shell

Thus protecting it from

The impact of the “bad” environment in the digestive tract

Especially infants and young children

Their intestinal flora is not yet complete

The immune system is not yet fully developed

Once the virus invades the intestine

Stool is very likely to cause diarrhea

(The three-layer protein shell of rotavirus, drawn by @Zhao Bang/Planetary Research Institute)

▼

The embarrassing thing is

For adults

HIV can also enter through the digestive tract

But from the "opposite direction"

There is almost no obstacle in this direction

Because they chose anal sex.

During anal sex

The rectal epithelial tissue can be easily torn

HIV breaks in

No Man's Land

This is the gay population

We need to be alert to the reasons why AIDS spreads

(Schematic diagram of the human rectum structure, drawn by @Zheng Borong/Planetary Research Institute)

▼

After talking about the skin, eyes, urogenital tract and digestive tract

The next 5th invasion direction

It is the one that viruses like most.

Right now

respiratory tract

A large number of viruses

Choose Invade from here

(Illustration of major respiratory viruses, drawn by @郑伯蓉/Planetary Research Institute)

▼

Taking the lungs as an example

Human lungs have about 300 million terminal alveoli

When unfolded, the total area is up to 100 square meters

The inhaled gas is in this 100 square meters

Full contact and exchange

The probability of being infected by a virus

Greatly increased

(Schematic diagram of human lung structure, drawn by @Zheng Borong/Planetary Research Institute)

▼

But the respiratory tract is not defenseless.

Its surface is covered with cilia

There is also a layer of mucus wrapped

Some virus particles are trapped in mucus

Pushed into the throat by cilia

Immediately swallowed

or coughed up

Losing the chance to invade

(Schematic diagram of respiratory epithelial tissue structure, drawn by @Zheng Borong/Planetary Research Institute)

▼

Skin, eyes, urogenital tract

As well as the digestive tract and respiratory tract

These are the five major invasion directions of the virus

Understand these

I guess you already understand

Your body is already infected with the virus

Set up a heavy line of defense

As long as you pay attention to protection

and improving personal hygiene and living habits

There are still many opportunities to prevent the virus from invading

Your enemies are powerful, insidious, and cunning.

But you know yourself.

There is a way to defend against the enemy

(The five major directions of virus invasion, drawn by @郑伯蓉/Planetary Research Institute)

▼

However

The you before

Maybe you didn't realize this.

A violent pleasure will lead to a violent end

The virus still rushed into your body

It's time to start

Next defense system

Innate immune system

Come on, beasts.

Round 2

Humanity strikes back

Innate immune system

The reason why it is called "innate"

Because it is the product of millions of years of human evolution.

For each human member

Innate defense system

The system is mainly composed of three armies

When the virus enters the body

Attacked cells immediately sound an "alarm"

Afterwards

A "security force" appeared on the scene

Right now

Interferon warning system

Interferon is a signaling protein

It is produced inside infected cells

Can inhibit the synthetic function of the cell

Prevent viruses from using them to replicate themselves

(The picture below shows a cell infected with the cowpox virus. The circle marked with blue fluorescence is the nucleus of the host cell, and the blue area next to the nucleus is the "virus factory" where the virus replicates and assembles. The picture comes from @VCG)

▼

on the other hand

It is also transported to the cell surface.

Sends a warning to surrounding cells

“You are likely to be invaded”

"Please be prepared to turn off the synthesis function at any time"

“So as not to be exploited by viruses”

at the same time

The Second Army

The real "attack force" also began to move out

Right now

Macrophages

As the name suggests

The cells responsible for "swallowing" harmful substances such as viruses

(Schematic diagram of the main phagocytes in the human body, drawn by @赵榜/Planetary Research Institute)

▼

in

Macrophages

It is the big eater in the phagocyte family.

They are usually in

The "resting" state of "wandering around and occasionally eating"

But when the virus appears

Macrophages become active as if they were electrocuted

Devour the Intruder

(Macrophages are engulfing bacteria, for illustration only, scanning electron microscope imaging, picture from @NIAID)

▼

If there are too many intruders

Macrophages also recruit other phagocytes

Come to help

Neutrophils

It is the largest force

Neutrophils in adults

The total force can reach 20 billion

(Pathogens are captured by neutrophil extracellular traps, scanning electron microscope imaging, image from @NIAID)

▼

In addition to the "early warning force" and "attack force"

Considering that many viruses lack "grip" on their surfaces

Not conducive to phagocyte engulfment

The innate immune system has been carefully organized

The Third Army

The "assisting force" responsible for supplementary support

Right now

Complement system

Complement proteins of the complement system are mobilized in large numbers

Sticks to the surface of the intruder

After some "modification"

Macrophages, neutrophils

It's easier to gobble up the virus

(Schematic diagram of the regulatory role of complement, drawn by @赵榜/Planetary Research Institute)

▼

but

Don't underestimate these assists.

Many times they will directly take action

By punching holes in the virus surface

Let the virus break down

It can be called "tearing alive"

Guard troops , attack troops , assist troops

The three armies work closely together

The virus caused heavy losses

But at this moment you

Also quite unhappy

Your nervous system

Start to raise body temperature

Increased body temperature once again accelerates immune cells

Migration to infected areas

This phenomenon is

fever

(A fever clinic in Daqing City, Heilongjiang Province, photographer @王理达)

▼

To deliver reinforcements to the battle site

Your blood vessels dilate and blood flow increases

Capillary congestion

Causes your skin to turn red

Even red spots

Increased vascular permeability

Cells and fluid in blood vessels

More easily penetrate into surrounding tissues

Causes tissue swelling

If you have nasal swelling

It will make you feel stuffy

Fluid seeping from the blood vessels in the nose

It makes your nose runny again

Cells that died in battle

It will turn into pus

(Runny nose is a common symptom among the general public. The picture below shows Brazil coach Tite blowing his nose during the 2018 Russia World Cup group match. The picture comes from @VCG)

▼

Other chemicals

It also stimulates nerve endings

Causes muscle pain

Redness, heat, swelling, pain

All of the above reactions are called

Inflammatory response

It's your body's way of fighting off invaders

When you suffer from inflammation

Victory seems to be in sight

But the devil is always stronger than the virtue.

Viruses are constantly improving their skills in the fight

Fighting the immune system

include

Replicate at an alarming rate

The only martial arts in the world that cannot be defeated is speed

Influenza virus and rhinovirus within 6-7 hours

In a cell

Thousands of copies

(The H1N1 subtype of influenza A virus caused the "Spanish flu" in 1918, killing more than 50 million people. Image from @VCG)

▼

Adenovirus can be found in a cell

Produces up to 100,000 virus particles

10-100 times more potent than most viruses

It can be called "infrastructure maniac"

Even the innate immune system

Many viruses have been eliminated

But the virus doesn't care about casualties

They are fanatics with the advantage of clustering

(Adenovirus, with an icosahedral structure, image from @VCG)

▼

There are some viruses

Treat others the way they treat you.

They found a way to interfere with interferon production

Such as measles virus , rotavirus

(Rotavirus, with a large number of spikes on its outer shell, image from @VCG)

▼

In conclusion

This is an inhuman invasion.

Any strategy of the virus

Both cruel and reasonable

Fortunately

When you are at a loss

Several "scouts" quietly came to your body's arsenal

A hidden defense system

Awakened

listen!

This is the roar of the adaptive immune system !

Round 3

decisive battle

The innate immune system has a remarkable “broad spectrum”

Its working method is

"Use the same weapons to defend against different viruses"

Although it can eliminate most invaders

But for special and rare

And the ruthless character who is "skilled and bold"

It seems powerless

Adaptive immune system

Change strategy

"Use different weapons to defend against different viruses"

Come one, hit one

Your physical body

It's that powerful

(Illustration of the characteristics of the two major immune systems, drawn by @Zheng Borong/Planetary Research Institute)

▼

It consists of two special forces

But because the lethality is too strong

If you are not careful, you may hurt yourself

Cause tissue damage and even endanger life

Therefore, the deployment of these two troops must have a certificate.

So, where do the credentials come from?

The "Scouts" mentioned above

The task of obtaining the certificate

They are composed of a small number of macrophages , dendritic cells , etc.

After "dismembering" some of the invaders on the front battlefield

Quickly withdraw from the battlefield

and take part of the invader's body

Reaching the Adaptive Immune System

This is the proof

(A phagocyte is delivering viral proteins to multiple T cells. This process is called "antigen presentation". The phagocyte is in the middle. The picture comes from @VCG)

▼

The first special forces unit to receive the certificate

T cells

Immediately examine the remains of the intruder

T cells mature in the thymus

Named after the first letter of the thymus gland

In our body

With 30 billion of these T cell special forces

They have different skills

Targeted killing of a specific virus

(Conceptual diagram of T cells attacking infected cells, gray is T cells, picture from @NIAID)

▼

After comparison

Those T cells that are most effective against the virus are selected

The special forces production factory started immediately

Using this T cell as a prototype

Mass production of new special forces

Killer T cells

(cytotoxic T lymphocyte, CTL)

Killer T cells are mobilized

Scanning the infected area cells one by one

Trying to find cells invaded by the virus

and kill them

(Three killer T cells are "besieging" the cancer cell in the middle. The blue mark is the cell nucleus, and the red mark is the vesicle containing killer chemicals, which is like a "kiss of death". The same is true for attacking virus-infected cells. The picture comes from @NIAID)

▼

At this time

Cells invaded by viruses also understand the righteousness

They release a protein on their surface

Calling on killer T cells

"Shoot at me"

then

Killer T cells release an enzyme

Injection into invaded cells

Thus "poisoning" the latter

Or help the latter start the "suicide program"

Die together with the virus inside the cell

Apoptosis

(Sketch of the working principle of killer T cells, drawn by @赵榜/Planetary Research Institute)

▼

Next

Phagocytes will die

Swallow the virus together

Clean and without any traces

Clears infected cells

But there are still a lot of virus particles outside the cells

This is when we need a second special forces unit.

B cells

Appeared

It matures in the bone marrow.

Named after the first letter of the English alphabet of the bone marrow

In our body

With 3 billion B cell special forces

Targeting different viruses

Develop different skills

(The pink ones in the picture below are B cells, imaged by scanning electron microscope, the picture comes from @VCG)

▼

When the B cell gets the certificate and compares

Certain B cells that are most effective in killing a certain virus are selected

Special Forces Production Factory Reopens

But this time the cells cloned in the factory

No longer rushing directly to the battlefield

Instead, turn yourself into a production machine

Producing proteins that specifically target a certain virus

Antibody

(B cells differentiate into plasma cells, which begin to produce large amounts of antibodies, which are the small Y-shaped particles in the picture. Conceptual image. Image from @VCG)

▼

Antibodies can bind to the surface of the virus

Make viruses lose their ability to invade cells

It can also help complement tag viruses

Facilitates phagocyte engulfment

Even through the placenta

Entering the fetus

Protecting vulnerable newborns

visible

Antibodies

The virus is basically dead.

(Antibodies attach to the surface of the virus in large quantities, rendering it incapable of infection, conceptual image, image from @VCG)

▼

Under the action of antibodies

The virus particles outside the cell were finally cleared

Now

Peace and tranquility inside and outside the cell

Your body has returned to normal.

But it’s not over yet

Some B and T cells have already prepared for a rainy day

They no longer produce antibodies and killer T cells

Instead, keep the corresponding "files"

For memory B cells and memory T cells

With them

When our bodies encounter the same virus again

The corresponding antibodies and killer T cells will be produced immediately

In the early stages of infection when you are barely aware of it

Remove the virus

This is also the case when people are infected with certain viruses

The reason why you won't get infected again

(Faced with the same virus, memory B cells can quickly produce a large number of antibodies; the picture below is a conceptual diagram, with B cells on the top and viruses on the bottom, the picture comes from @VCG)

▼

So far

I guess you already know

Physical barriers such as skin

Innate immune system , adaptive immune system

It constitutes the three-layer defense system of our human body.

Plus the "archive" of memory cells

Every time the human body experiences a virus attack

The immune system may get a boost

As the saying goes

“What doesn’t kill me makes me stronger”

It is this mechanism

For millions of years, humans

From the vast majority of virus attacks

Survive

This is the power of the human body.

(Schematic diagram of the three-layer defense system of the human body, drawn by @Zheng Borong/Planetary Research Institute)

▼

but

Such a system also has obvious bugs

Right now

time

Although the adaptive immune system is strong

But it usually takes a week or even longer

To produce enough antibodies and killer T cells

This gave the virus an opportunity to take advantage

They often

Before the adaptive immune system is activated

The attack has been completed

We don't have the corresponding memory cells formed in our bodies

The virus can infect the same person multiple times

(T cells, the "special forces" of the adaptive immune system, yellow is platelets, picture from @VCG)

▼

Furthermore

Human expansion into wildlife habitats

Animals that previously had little contact with humans

Contact Us

New virus

From these wild animals

Keep jumping to humans

Our immune system is overwhelmed

For example

One of the scariest viruses the world has ever known

Ebola virus has a mortality rate of over 50%

It may have jumped from fruit bats to humans.

(Fruit bats in Kasanka National Park, Zambia, photo from @VCG)

▼

HIV is well known

Its type 1 virus jumped from chimpanzees to humans

It has only been a few decades since it was identified

75 million people have been infected

today

Less than one in 200 people in the world

One person carries the virus

HIV can transfer its genes

Integrated into the human genome

And take over the human immune system

This makes it extremely difficult to eliminate the virus.

(Chimpanzee in Liberia, Africa, picture from @VCG)

▼

Even more serious is

Existing viruses undergo gene mutation , recombination and reassortment

Also constantly

evolution

We still don’t know enough about the world of viruses

What we know about our immune system

It's not enough

“Only when you climb high and look far away can you realize how vast the world is.

"When you put yourself in the vastness of the universe, you will realize how small your body is."

There is still a long way to go

Come on science! Come on humanity!

In the attack of the virus

Please make us stronger

(Photo by Zheng Borong/Planetary Research Institute)

▼

This article was created by

Written by: Director

Editor: Mr. Zhen

Design: Zheng Borong, Zhao Bang, Wang Shenwen

Photos: Xie Yuhan, Ren Bingxu

Reviewer: Fengzi

Expert review: Lv Xiang, Li Lei, Dong Dong

【References】

1. Lauren Sompayrac, Overview of Virology, Peking University Medical Press, 2016

2. SJ Flint et al., Principles of Virology, Chemical Industry Press, 2014

3. Zhou Deqing, Microbiology Textbook, Higher Education Press, 2011

4. Lauren Sompayrac, Overview of Immunology, Peking University Medical Press, 2016