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Animal Systems Terms

Get down with the lingo

Active Immunity

This type of protective defense system happens after we develop a disease (like chickenpox) or are exposed to a live pathogen (like with the measles vaccine). Our body does all the hard work and makes the antibodies itself, as opposed to buying the premade kind at the local pharmacy.

Adaptive Immunity

Only vertebrates have this type of immunity (yet another perk of living large with a spine), which kicks into gear if pathogens get past the innate defenses. Based on the pathogen's antigen, the cells within this adaptive immune system respond only to certain pathogens, but communicate with others to wipe out the invader.

Adrenal Cortex

Often lumped under the collective adrenal glands, the adrenal cortex is the part of the endocrine system that specifically releases the steroid hormones that are major players when it comes to the stress response.

Adrenal Medulla

The other half of the adrenal glands, the medulla releases neurohormones (or catecholamines), like norepinephrine and epinephrine, that tell the body how to react when you get yourself worked up in a tizzy.

Amino-acid Derived Hormones

These hormones are made up of a few tyrosine or tryptophan amino acids put together. The big ones here are epinephrine and thyroid hormone.


Made by the specialized B cells, these proteins recognize and "tag" pathogens for destruction. These guys hunt down the invader and shout from the rooftop about it.


Literally, an "antibody generator." These little bits of protein sequences are specific to a particular pathogen and allow specialized immune cells to recognize the pathogen. Think of them as the megaphone that lets the antibody scream the invader's location to the rest of the immune system.

Autoimmune Disease

No one is perfect, not even the immune system. And sometimes it screws up and attacks something it isn't supposed to. In autoimmune diseases, a "normal" protein is mistakenly seen as foreign by certain immune cells, and they attack it.

Autonomic Nervous System

This is a part of the peripheral nervous system that controls, well, the autonomic parts of our everyday existence, like a beating heart and digesting lunch—the stuff we don't even have to think about. It frees up more time to think about important stuff, like what we think about Justin Beiber's new haircut.


A long stringy part of a neuron. Axons are like the telephone wires that send information to other neurons.

B Cells

Part of the humoral immune response, B cells are activated by particular antigens. When they become active, B cells make antibodies and daughter cells to ramp up the attack response.

Bone Marrow

This is where all immune cells are born as stem cells. While some leave, B cells do not, opting instead to hold down the fort and stay here to mature.

Cell-mediated Immune Response

A type of defensive response within the adaptive immunity umbrella. The big gun immune cells here are the T cells.

Central Nervous System (CNS)

The decision maker of the nervous system. The brain and the brain stem fall into this part of the body.

Cerebrospinal Fluid

The "brain's blood." This is the fluid that makes sure all the neurons in the brain are hunky dory. It provides them with nutrients, but also prevents the brain from hitting against our skulls when we dance.

Constitutive Secretion

Hormones are only made and released when they are needed. It's like a fancy restaurant where they actually make the food after you order it. No freezing and thawing here.


Chemicals released by many types of leukocytes that communicate with and activate other cells.

Cytotoxic T Cell

These are the head honchos on Team T Cell. When they find their match, they release their potent cell-killer chemicals to ward off infected cells and unwanted bad guys.


This is the place where a neuron receives all incoming information. Dendrites look like tree roots that extend from the cell body, and like tree roots, they're there to suck up all sorts of messages sent by neurotransmitters.

Effector Cell

This is a generic term for any lymphocyte that has matured into a specific type of cell that mounts an immune response. If it's a B cell, it produces antibodies, and if it's a T cell, it might be of the cytotoxic variety, getting down and dirty and actually destroying cells.

Endocrine System

The body's way of using hormones to communicate messages throughout the body—our own microscopic version of tin cans and string. Hormones are released from ductless endocrine glands, and travel through the body to an end organ, where the hormone will evoke some type of response.

Enteric System

This part of the autonomic nervous system allows the brain to communicate with the digestive system. Although the enteric system is its own system, it's not its own boss, since other parts of the autonomic system tell it when it can and can't function.


These immune cells have tiny packets of cell-killing chemicals inside them. When a pathogen is consumed, these packets burst, and the chemicals act to kill the invader.

Helper T Cell

As a behind-the-scenes type of T cell, helper cells give a helping hand wherever one is needed. They might mature into memory cells or recruit other immune cells to the fight, but rarely do they receive any credit.

Humoral Immune Response

An antibody-mediated immune response within the adaptive immunity umbrella. B cells are the enforcers here, trying their darndest to keep the body happy and healthy.


This is the nagging, bossy endocrine gland that dictates what the pituitary gland can and cannot release. Since it's also a part of the brain, the hypothalamus is really important in the communication between the nervous and the endocrine systems.

Immune System

An animal's defense mechanism against all the nasty stuff that makes its way into the body. The immune system employs both the innate and adaptive responses to help it ward off infection.

Innate Immune Response-

This is the fast-acting team of pathogen killers that is always on the job, working to guard against foreign invaders. It includes barriers, like skin and mucus, as well as special cells that devour infected cells. All animals, vertebrate and invertebrate, have innate immunity.


Part of the innate immune system, these peptides are made by virally-infected cells to warn neighbors about the incoming threat. It helps other cells get ready for the attack, and ultimately resist the virus.


These are the white blood cells, the foot soldiers of the immune system. They grow up in the bone marrow, but when they're ready, they're sent off into the bloodstream to start hunting for bad guys.


These are white blood cells of the adaptive immune system that circulate throughout the blood and lymph. Both B and T cells are lymphocytes.


Giant phagocytic cells found in the spleen and lymph nodes. These cells are super hungry, and devour pathogens.

Major Histocompatibility Complex (MHC)

When a healthy cell becomes infected, this group of proteins binds to specific antigen fragments, and brings them to the surface – raising the flag, if you will. It's this MHC and antigen fragment that the immune T cells recognize to kill the infected cell.

Memory Cell

These lymphocytes develop in response to a current infection, and hang around so that if the pathogen ever comes back, the immune response is faster and stronger. Don't make a memory cell mad – they hold grudges.

Natural Killer Cells

As a part of the innate immune system, these cells directly kill infected cells. When they find a wayward cell (one that is infected with a snotty-nosed virus, perhaps), they attach and inject the bad cell with killer chemicals.


Bundles of axons and dendrites that move information through the nervous system, letting the brain and body communicate.


Chemicals that are involved in endocrine and neuronal communication. They are released from the adrenal medulla when an animal finds itself in a stressful situation and get right to work on trying to fix it.


Cells of the nervous system that receive and send out information to each other. Each neuron talks to another with their long axon, and receives information through their branched dendrites. These guys are the telephone wires of the body's nervous system, blasting messages far and wide faster than you can blink.


The brain's way of communicating with the rest of the body. Think of it as one massive game of "telephone," except the point is for the message to make it through intact.


Like the words we use to tweet, these are the chemicals that transmit information from one neuron to another. There are many different types of neurotransmitters. They each have their own particular receptor, and each has a different response when it binds.

Parasympathetic System

This is the part of the autonomic nervous system that's cool and collected. It slows our heart rate and stimulates digestion when all is calm.

Passive Immunity

With passive immunity, the body doesn't have to do much of anything to be protected from infection. The antibodies themselves are somehow introduced, either through a mother's milk or injection (like with snake-bite antivenom). The body is perfectly happy sitting by and letting someone else do all the hard work.


The ski-masked bank robbers, the aliens with three heads, those nasty jumping spiders—however you want to think of them, pathogens are the bad guys that invade and attack our bodies. They could be green with alien antennae and three eyes, but are more likely bacteria, fungi, toxins, protozoa, parasites, or cancer cells.

Peptide Hormones

These hormones are made up of multiple peptides (lots of amino acids), and are the most popular hormones of all. Our favorite peptide hormone is insulin.

Peripheral Nervous System (PNS)

This system is charged with carrying information to the brain (via sensory nerves) and away from the brain (via motor neurons). It controls all types of nervous system messages, both conscious (like running) and unconscious (like breathing). It calls all the shots on one important game of phone tag.


The devouring and digesting of pathogens by a host cell. Sort of like an alligator that eats just about anything, phagocytes aren't particular about the type of pathogen and devour the bad guys when they see them. Once they trap the pathogens, phagocytes bring them into the cell and break them down so they can't do any more damage.

Pituitary Gland

One of the master glands in the endocrine system. The pituitary often receives direction from the hypothalamus on whether to release hormones or not. This gland is responsible for the synthesis and release of growth, thyroid-stimulating, follicle-stimulating, and luteinizing hormones.

Regulated Secretion

The grocery stores of the endocrine system where hormones are synthesized and stored well-ahead of time. When the body senses a need for that hormone, there's no mixing and baking here, the hormones are simply released.

Somatic System

Part of the peripheral nervous system that controls the skeletal muscle and external sensory organs like the skin. When your brain decides that your nose needs a good picking, it gets the nerves of the somatic system to activate your muscles that do the actual picking. Also, the ones that turn your head to make sure no one is looking.

Steroid Hormone

A fat-soluble hormone that squeezes through cell membranes and binds to receptors at the nuclear membrane. They all start out as cholesterol rings, but in the end, all steroid hormones are chemically distinct, like estrogens and corticosteroids.

Suppressor T Cell

These T cells are the stoplights to the immune response. They monitor the response closely, and when it gets out of hand and the immune traffic is threatening to cause major gridlock, suppressor T cells limit the immune response and tell everyone to chill out.

Sympathetic System

Within the autonomic nervous system, the sympathetic system kicks in at those stressful moments in life. Nerves in this system talk to involuntary muscles and ramp up heart rate and blood pressure when a grizzly bear chases you down the street, for example.


The physical location for neurotransmission. A neurotransmitter hops across this synaptic gap and sends a message through its receptor.

T Cells

Although not a true part of alphabet soup, these cells directly regulate the immune response when they see just a snippet of antigen on the surface of a host cell.

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