Explainer: The body’s immune system

Meet the army of cells that make up the body’s health-protection squad

Your immune system is probably something you ignore, at least until you get ill. Then you realize how important the immune system is. It’s all the various organs, cells and proteins spread throughout the body that protect us from bacteria, viruses and other potentially harmful invaders.

Cells of the immune system can be split into two closely related military squads: innate and adaptive. Troops belonging to the first — innate — patrol the body to detect intruders, such as bacteria and viruses. These troops don’t trust anyone, not even their own body’s cells. But they don’t have to fend off bad guys alone. When faced by a tough adversary, they can call in back-ups — the adaptive troops — that are skilled in even heavy combat.

Naama Geva-Zatorsky works at the Rappaport Technion Integrated Cancer Center in Haifa, Israel. There, she studies microbes and the immune system. The mission of the body’s innate immune system, she explains, is to distinguish between friendly cells (the body’s own cells) and intruders (non-self). Friendlies have specific structures on their surface, like a flag, that the innate troops recognize. They know to ignore these cells. Intruders lack those familiar surface “flags” found on the body’s healthy cells.

When innate troops detect “non-self” structures — such as a virus — they set off alarms. These call out other troops to help eliminate the intruders as quickly as possible. The three most important types of innate troops are immune cells known as neutrophils (NEW-troh-fils), macrophages (MAK-roh-faeges) and dendritic cells.

Neutrophils survey their neighborhood by “tasting” microbes. When they find an intruder, these troops release small signalling molecules called cytokines (SY-toh-kynes). Cytokines quickly recruit help to the developing fight. They tell other immune cells what type of help they need and where to send it. Sometimes neutrophils also change shape. They sprout long arms and form a web-like net to trap invaders.    

Macrophages are bigger, curly-shaped cells that respond to the neutrophil alarms. They hang out in the tissues longer than the neutrophils do. While there, they gobble up as many invaders as possible through a process called phagocytosis (Fag-oh-sy-TOH-sis). Macrophages won’t stop eating until nothing is left. 

Dendritic cells arrive around the same time as macrophages. Dendritic cells digest pieces of microbes and then show them out on their long arms. In this way they recruit a back-up squad into the battle: the adaptive immune system.

The back-up forces

The heavy forces of the adaptive immune system don’t get involved with every little invader. Most of the time, innate immune cells can win the battle by themselves. We don’t even notice it happening. However, when worrisome pathogens invade our bodies, cells of the adaptive immune system take over. They tailor their particular response to each invader but need a few days to make much headway. 

Sometimes intruders sneak into the body and take over some healthy cell. That’s where it will multiply (or replicate). But once inside, that invader is also hidden. The innate cells can no longer find it.

Helper T cells, a type of white blood cell —or lymphocyte — now step in. They collect info on the enemy, regardless of whether those attackers are inside of a cell or out. Then they’ll pass this intel along to another team, the killer T cells.

Killer T cells are another type of lymphocyte. They can kill anything that looks suspicious.

B cells release weapons called antibodies that seek out the enemy. Antibodies, which are families of Y-shaped proteins, are sticky. They glom onto everything that resembles the intruder. In many cases this will be the actual invader. Other times it might be pieces left behind when the intruders are killed. They might even be invader look-alikes created by vaccines. These invader mimics will allow B cells to respond quicker if ever a real microbial invader comes along.

The antibodies tag their target cells so that other immune-system teams can later go in and take them out. Antibodies will also stalk escaping enemies throughout the body. Those antibodies seek out surface patterns on cells or cell bits that identify specific intruders.

After a battle is over, invader-specific B cells remain behind. They form a pool of veterans. They preserve a memory of the former invasion. Based on that living memory, they’ll be able to help the body react faster and better the next time the same type of invader arrives. This process is called immunological memory and it’s the key to how vaccines work.

“When an invader comes in the body, it’s great to have an alert immune system,” says Naama. “But it’s also important that it doesn’t overdo.” There are several ways to stop such an overblown response.

Regulatory T cells, for instance, tamp down the activity of other T cells, before they get out of control. During a skirmish, T cells can get so keyed up that they risk getting out of control. That’s where T-reg squads come in. They help calm down the T-cell combat troops so that the immune system can return to normal.

The immune system helps keep us safe. We can also exploit it against deadly diseases thanks to vaccines and immune-boosting drugs. “The immune system is cool, but we need to keep it healthy,” says Naama. How? Look after yourself. A healthy body means a healthy immune system. 

This video explains how cells in the innate and adaptive immune squads cooperate to fight invading pathogens — such as disease-causing microbes.

Power Words

agent: A person or thing (it can be a chemical or even a form of energy) that plays some role in getting something done.

antibodies: Any of a large number of proteins that the body produces from B cells and releases into the blood supply as part of its immune response. The production of antibodies is triggered when the body encounters an antigen, some foreign material. Antibodies then lock onto antigens as a first step in disabling the germs or other foreign substances that were the source of those antigens.

B cell: A type of small white blood cell (also known as a B lymphocyte), which plays an important role in the immune system. Made in the bone marrow, these cells mature into plasma cells, and serve as the source of antibodies.

bacteria: (singular: bacterium) Single-celled organisms. These dwell nearly everywhere on Earth, from the bottom of the sea to inside other living organisms (such as plants and animals). Bacteria are one of the three domains of life on Earth.

cell: (in biology) The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.

cytokine: A small protein secreted by certain cells of the immune system which the body uses to have some particular controlling effect on other cells. Examples include interferons, interleukins and growth factors.

dendritic cell: A type of immune system cell that initiates the primary response to a foreign substance.

digest: (noun: digestion) To break down food into simple compounds that the body can absorb and use for growth. Some sewage-treatment plants harness microbes to digest — or degrade — wastes so that the breakdown products can be recycled for use elsewhere in the environment.

force: Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.

germ: Any one-celled microorganism, such as a bacterium or fungal species, or a virus particle. Some germs cause disease. Others can promote the health of more complex organisms, including birds and mammals. The health effects of most germs, however, remain unknown. 

honey: A viscous (gooey) material that honeybees store in their honeycombs. The bees make it from nectar.

immune: (adj.) Having to do with immunity. (v.) Able to ward off a particular infection. Alternatively, this term can be used to mean an organism shows no impacts from exposure to a particular poison or process. More generally, the term may signal that something cannot be hurt by a particular drug, disease or chemical.

immune system: The collection of cells and their responses that help the body fight off infections and deal with foreign substances that may provoke allergies.

innate: An adjective for some behavior, attitude or response that is natural, or inborn, and doesn’t have to be learned.

macrophage: A type of white blood cell dispatched by the immune system. Like janitors of the body, they gobble up germs, wastes and debris for disposal. These cells also stimulate other immune cells by exposing them to small bits of the invaders.

microbe: Short for microorganism. A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.

molecule: An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types.

neutrophil: A type of white blood cell released by the immune system. It gobbles up wastes and release chemicals that can digest cells, including germs.

pathogen: An organism that causes disease.

primary: An adjective meaning major, first or most important.

protein: A compound made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. Among the better-known, stand-alone proteins are the hemoglobin (in blood) and the antibodies (also in blood) that attempt to fight infections. Medicines frequently work by latching onto proteins.

recruit: (verb) To enroll a new member into some group. It could be into the military. Or it could be into participating in a research group to test some drug, behavior or environmental condition.

replicate: (in biology) To copy something. When viruses make new copies of themselves — essentially reproducing — this process is called replication.

risk: The chance or mathematical likelihood that some bad thing might happen.

survey: To view, examine, measure or evaluate something, often land or broad aspects of a landscape.

SWAT: Acronym for special weapons and tactics. It’s a term for the type of heavily armored, combat troops who can support or even take over for patrol officers and detectives in challenging conditions.

system: A network of parts that together work to achieve some function. For instance, the blood, vessels and heart are primary components of the human body’s circulatory system. Similarly, trains, platforms, tracks, roadway signals and overpasses are among the potential components of a nation’s railway system. System can even be applied to the processes or ideas that are part of some method or ordered set of procedures for getting a task done.

T cells: A family of white blood cells, also known as lymphocytes, that are primary actors in the immune system. They fight disease and can help the body deal with harmful substances.

tag: (in immunology) A chemical change that allows the immune system to identify cells or other material that it should attack and disable or remove.

vaccine: (v. vaccinate) A biological mixture that resembles a disease-causing agent. It is given to help the body create immunity to a particular disease. The injections used to administer most vaccines are known as vaccinations.

virus: Tiny infectious particles consisting of genetic material (RNA or DNA) surrounded by protein. Viruses can reproduce only by injecting their genetic material into the cells of living creatures. Although scientists frequently refer to viruses as live or dead, in fact many scientists argue that no virus is truly alive. It doesn’t eat like animals do, or make its own food the way plants do. It must hijack the cellular machinery of a living cell in order to survive.