Adaptive immune system

The adaptive immune system is a vital part of how our bodies fight off germs and stay healthy. It is one of two main ways that animals with backbones, like humans, protect themselves from diseases. Unlike the other system, called the innate immune system, the adaptive immune system can remember specific germs it has fought before. This helps the body respond faster and stronger if the same germ tries to infect us again.

This system uses special types of white blood cells called lymphocytes, mainly B cells and T cells. B cells make proteins called antibodies that can stick to harmful germs, stopping them from hurting our bodies. T cells help in other ways, like destroying infected cells. When the adaptive immune system fights a germ for the first time, it creates a kind of memory. This means that if the same germ shows up again later, the body can fight it off more quickly.

Because of this memory, vaccines are possible. Vaccines are like practice runs for our immune system. They show the adaptive immune system what a germ looks like without causing the disease, so the body can learn to fight it. This is why many people are protected from diseases like measles for their whole lives after being vaccinated or getting the illness once. The adaptive immune system is very flexible, able to create many different ways to recognize and fight new germs, keeping us safe from many infections.

Naming

The word "adaptive" was first used by a scientist named Robert Good in 1964. He used it to describe how the body makes special proteins called antibodies to fight off germs. Good liked this word because it suggested that the body could change and adjust to fight different germs.

Later, scientists began using "adaptive" to talk about a wider range of immune responses, including some that don’t involve changing genes. Today, most books use "adaptive" to mean the same thing as "acquired" immunity, which is the body’s way of specifically targeting and remembering germs it has fought before.

Functions

When a harmful germ gets past the body's first line of defense, the adaptive immune system kicks in. It helps the body recognize and fight off specific germs. This system creates special cells that remember these germs, so the body can respond faster if they show up again.

The adaptive immune system has three main jobs:

• It can tell the difference between the body’s own cells and foreign germs.
• It creates responses to remove the germs or infected cells.
• It builds memory so the body remembers the germs for future protection.

In people, it usually takes about 4 to 7 days for this system to start working well against a new germ.

Main article: antigen presentation

T and B lymphocytes are special cells that help our bodies fight germs. The human body has about 2 trillion of these cells, which make up 20–40% of all white blood cells. Most of these cells move around in tissues and the lymphatic system, which includes the lymph nodes and spleen. Only a small amount is in the blood.

B cells and T cells come from the same basic stem cells and look the same until they are needed. B cells help create substances that fight germs, while T cells directly attack infected cells. In most animals with a backbone, these cells are made in the bone marrow, and T cells then go to the thymus to grow more.

In our bodies, there are three main types of these cells: ones that have not yet met a germ, ones that are currently fighting an infection, and memory cells that remember past infections to help us faster next time.

Antigen presentation

Main article: Antigen presentation

The adaptive immune system helps the body tell the difference between its own cells and harmful invaders. Our cells show special markers called "self" antigens. When immune cells find markers from outside invaders like bacteria or viruses, this triggers a response.

Certain immune cells, such as dendritic cells, B-cells, and macrophages, are especially good at showing these foreign markers to other immune cells. They help prepare the body's defense by presenting these markers using special molecules called MHC. This helps T-cells recognize and fight different kinds of threats.

Dendritic cells catch outside invaders, break them into pieces, and show these pieces to T-cells in special areas of the body. Inside cells, viruses and bacteria also create markers that are shown to T-cells, helping to stop infections.

T lymphocytes

Main article: T cell

CD8+ T lymphocytes and cytotoxicity

Main article: Cytotoxic T cell

Cytotoxic T cells are a special group of T cells that help the body fight off infections. They can find and destroy cells that have been taken over by viruses or other harmful agents. When these cells spot an infected cell, they release special chemicals that break open the infected cell, stopping the spread of the infection. After the infection is gone, some of these cells stay around to help protect the body faster if the same infection comes back.

Helper T-cells

Main article: T helper cell

Helper T cells are important managers of the immune system. They don’t fight infections directly but tell other immune cells what to do. When they notice a problem, they send out signals that help activate other immune cells to do their jobs. There are different kinds of helper T cells that specialize in fighting different types of germs. Some focus on helping the body fight germs inside cells, while others help with germs outside cells.

Th1 and Th2: helper T cell responses

There are two main types of helper T cell responses. One type helps fight germs that live inside cells, like some viruses and bacteria. The other type helps fight germs outside cells, like certain parasites. After an infection, a few helper T cells remain to remember the germ and help the body respond faster if it sees the germ again.

Gamma delta T cells

Main article: Gamma delta T cell

Gamma delta T cells are another type of T cell that have unique ways of recognizing threats. They can act like both helper and cytotoxic T cells and can respond quickly to certain signals from microbes or stressed cells. These cells are important for bridging the body’s immediate defenses with its longer-term immune memory.

B lymphocytes and antibody production

Main articles: B cell and Humoral immunity

B cells help make special proteins called antibodies that travel in our blood and lymph. These proteins help our body fight germs by recognizing and stopping them. There are different kinds of antibodies, each designed to handle different types of germs.

When B cells find a germ they match, they turn into factory-like cells called plasma cells. These cells make lots of antibodies that stick to the germ, making it easier for other immune cells to find and destroy it. Some of these B cells become memory cells, which remember the germ so the body can fight it faster if it shows up again.

Alternative systems

Main article: Adaptive immunity in jawless vertebrates

Some animals, like lampreys and hagfish, have a special way to fight off germs even though they do not have the same immune system as other animals. These animals use special molecules called variable lymphocyte receptors (VLRs) to recognize and fight germs, similar to how other animals use antibodies.

Main article: CRISPR

Even small creatures like bacteria have ways to protect themselves. They use a system called CRISPR, which helps them remember and fight off viruses that try to attack them. CRISPR is part of their genetic code and acts like a memory bank against harmful viruses.

Immunological memory

Further information: Immunity (medical)

When certain cells in the body called B cells and T cells become active, some of them turn into memory cells. These memory cells remember how to fight off germs the body has faced before. If the same germ tries to infect the body again, these memory cells act quickly to defend the body. This makes the body's defense stronger and faster the next time it meets the same germ.

Babies are born with some protection from their mothers. While inside the mother, babies receive special proteins called antibodies that help fight germs. Breast milk also contains antibodies that protect babies until they can make their own. This is called passive protection because the baby’s body does not make these antibodies itself.

Active protection happens when the body learns to fight a germ after being exposed to it or through vaccines. Vaccines help the body learn to fight germs without causing the sickness. They work by showing the body a small, safe part of the germ so the body can prepare its defenses. This is how vaccines help keep people healthy.

Immunological diversity

The immune system can recognize and fight many different germs. It does this by making special proteins called antibodies and using special cells. Even before we meet a germ, our body can make trillions of different antibodies. This is possible because our cells mix and match small pieces of genes to create many different types of antibodies.

The immune system has two parts that work together. The adaptive part, which includes B cells and T cells, needs help from the innate part to work well. Together, they protect us from harmful germs.

Acquired immunity during pregnancy

The immune system normally knows the difference between what belongs to the body and what does not. This makes it interesting to learn how a developing baby, which the immune system might see as "not belonging," is protected.

One reason is that the baby grows inside a special part of the mother’s body called the uterus, which the immune system does not often check. Another idea is that the baby might help calm the mother’s immune response, maybe by using nutrients in a special way. More recent studies show that certain proteins in the uterus during pregnancy help control the immune response there.

In mammals that give birth to live young, certain viruses called endogenous retroviruses become active during early development. These viruses may help protect the developing baby from the mother’s immune system. They also help form a special tissue called the syncytium, which limits the movement of certain cells between the baby and the mother. Scientists think these viruses were part of the evolution of these mammals, helping babies survive inside the mother.

The human genome project discovered many of these viruses in our DNA.

Immune network theory

Main article: Immune network theory

Immune network theory helps explain how the adaptive immune system works. It focuses on how special parts of antibodies, called idiotypes, interact with other parts that react to them, called 'anti-idiotypes'. This theory was developed starting in 1974 by Niels Jerne and Geoffrey W. Hoffmann. It helps scientists understand diseases like HIV pathogenesis and work on creating an HIV vaccine.

The immune system has two main parts: one that works directly and another that remembers past invaders. When the body first fights off a germ, it uses quick, general defenses. These defenses then help prepare the adaptive immune system to fight the germ better next time. For example, in areas where malaria is common, children often develop strong defenses by school age. These defenses help control the germs that cause malaria and prevent serious illness.

Evolution

The acquired immune system developed from the innate immune system. Some simple sea creatures have cells that are part of this system. Even very early animals could tell apart their own cells from foreign ones.

A special animal called amphioxus, which is related to all vertebrates, has many parts that are used in the adaptive immune system. However, it does not have all the advanced features found in modern animals.

The acquired immune system, as we know it best in mammals, first appeared in jawed fish around 500 million years ago. These fish have many of the same immune cells and molecules that we have. Their immune systems became more complex partly because of changes in their DNA.

Jawed fish have a slightly different way of organizing their immune genes compared to bony fish, like mammals. This means they can create fewer variations in their immune response.

Jawless fish, another type of vertebrate, also have an adaptive immune system. It works in a different way, using special receptors to recognize foreign substances and rearrange their DNA to create diversity in their immune response.

Types of acquired immunity

The body can gain protection from sickness in two main ways: actively or passively. Active immunity happens when a person is exposed to harmful substances and their immune system fights them off. Passive immunity occurs when antibodies, which are special proteins that help fight sickness, are transferred from one person to another.

There are four ways this can happen, either naturally or through medical help. Natural active immunity happens when someone gets sick from exposure to a germ and then gets better. Natural passive immunity is when a mother passes antibodies to her baby through the placenta or breast milk. Artificial active immunity is achieved through vaccines, which introduce weakened or dead germs to help the body learn to fight them. Artificial passive immunity involves giving antibodies from someone else who is already protected against a disease.