This is the second of a two-part series on the immune system.

Lyrics are immediately below the video.


Welcome to immune system rap part two
Now we’ll learn what the specific responses can do,
When innate defenses start to be breached,
Antibodies and Killer T cells are unleashed

The specific response has three distinct components
Recognize, overcome, remember your opponent.
It’s acquired, adaptive, improving over time.
It lets you beat back pathogens it keeps you in your prime

The cells that carry out specific immunity,
Are known as lymphocytes they go by letters B and T
They’re often found in lymph nodes, your body fluid’s filters
You feel them get swollen when your system gets off-kilter

These specific responses are a vertebrate feature,
Not found in bugs or worms or any lacking-backbone creature
The response is elicited by what’s called an antigen
A portion of a molecule on an invading pathogen

“Antigen” means “antibody generator”
Polysaccharides or proteins on the cells of an invader
Antibodies are proteins B lymphocytes secrete
To bind with unique antigens and lead them to defeat

Both Bs and Ts can recognize specific pathogens
Through receptors on their membranes that bind with antigens
A B-cell receptor’s made of four polypeptide chains,
With forms a kind of Y the stem is stuck into the membrane

Two of the chains are heavy, two of the chains are light
The two tips of the Y form the antigen binding sites.
Activated B cells secrete their receptors,
As antibodies they’re our system’s number 1 protector

Random recombining within B and T receptor genes
Generates amazing receptor shape diversity
The staggering assortment of B and T receptors,
Lets us recognize specific invaders when they enter

T receptors are quite different but their function is the same.
They can bind with almost any antigen that can be named,
They’re just two chains, and right at their tip
Is the specific shape that binds invaders in their grip

T cells come in two main varieties.
The Helper T’s the general in the immune system army,
And cytotoxic, killer T cells: they’re assassins
They kill those zombie cells controlled by viral pathogens
Let’s examine how our B cells respond to an infection.
The big picture here’s called clonal selection.
A B-cell receptor binds an antigen with matching shape
This activates the B cell which divides and makes

A clone of plasma cells whose job is to secrete,
Antibodies for that specific antigen they’ll meet
Antibody action through our body fluids is called humoral.
We’ll see below the ways it send invaders to their funerals

B cell activation starts when phagocytic cells
Dendritic cells or macrophages, all these sentinels
Devour a pathogen, then break it down
Then hold it up as antigen showing what they found

The antigen’s displayed on special protein
Part of a family called MHC
“MHC’s” for “Major Histocompatibility
Complex,” it’s used to say “this is me I’m on your team

And here’s a little something, that’s trying to infect us.”
We gotta do something, or this’ll be the end of us.”
Our phagocytic sentinels use MHC class 2,
And holding up their antigen they make their move

To lymph nodes, where they bind with a Helper T
Whose receptor is also complementary,
To the antigen that the phagocyte’s displaying
This is the connection for which we’ve been praying

The Helper T grabs on with its CD4 protein,
Which connects antigen presenting cell and Helper T
And now the two cells, messages exchange
Inducing the Helper T to change

The messages are in the form of cytokines,
Secreted proteins that are used to define
other cells’ function, and even their form,
In this case they induce a Helper T swarm

Helper T clones itself into identical versions
Each of which has the receptor for the antigen
These helper Ts wait in a lymph node till they find,
A B cell to whom the same antigen did bind

Which means both B and Helper T have receptors
That complement the shape of this antigen infector,
Cytokines from Helper T get B cell to start division.
Makes a plasma cell army that’s devoted to the mission

Of secreting antibodies 2000 per second per cell
These bind with antigens on pathogens which feel like hell
Was unleashed as antibodies act like a tag
So that phagocytes can put those germs inside the bag

It’s called opsonization, an antibody trick,
Here’s another antibody move that’s pretty sick
By binding viral proteins antibodies prevent
Viruses from binding to cells they might infect

Gumming up these viruses is neutralization.
Another immune system way of pathogen negation
Antibodies also boost the complement system
Infected cells and germs explode they’re antibody victims!


Killer T is an assassin, not to be under-rated.
These cytotoxic killers do a cell mediated response,
Like Bs, Killer-Ts, need prior activation
By a phagocytic cell that makes a presentation

Of antigen displayed in an MHC protein
And often Helper Ts assist this activation scene
Once activated Killer Ts clone themselves into a platoon
That roams the body to bring about the doom

Of body cells with cancer or an infection,
And here’s how Killer T’s do target detection
Their T cell receptor binds with the antigen
Held aloft by tissue cells which are infected

The antigen’s held up in their class one MHC
The sickly tissue cell’s, saying “look what inside of me!
So the killer T grabs hold with protein CD8
And then proceeds to assassinate

The infected cell, it’s a mercy killing
Using perforins for membrane drilling
And granzymes to induce apoptosis
Infected cells die from these lethal doses

Remembering’s the last step in immunity
Allowing you to face repeat invaders with impunity
B and T memory cells are the trick
That build up your immunity – keeps you from getting sick.

Every time your specific responses have a victory,
There’s not only effector cells there’s also cells for memory
These memory cells stay behind with their receptors
Waiting to bind the same disease-causing vector

So in the next infection you skip all the preliminaries,
And go right to cloning plasma cells and even Killer-Ts
This mobilization means you’ll bring germs to their knees
You’ll wipe them out before you feel the symptoms of disease!

That’s why the first immune response you have is rather weak,
You beat the pathogen but mobilizing took you weeks
But the second response you have can really be fine!
Many, many, many more antibodies made much less time!

And that’s how vaccination works to keep away disease,
Injecting harmless antigens to develop memory
Cells with receptors that detect and arm
The specific immune response that keeps us from harm