Let's now move to the adaptive immune system.
This is a much more sophisticated, much more complex,
and much more important immune system for the defense of the animal body.
Every animal has it.
If you don't have it, you die.
Now, in looking at adaptive immunity,
the key thing to remember is that there are two major groups of microbial invaders.
We have the bacteria plus some protozoan,
a few worms that live outside cells.
They grow in tissue spaces.
Then you also have intracellular invaders such as the viruses,
some bacteria, and few protozoa that get inside cells.
The body needs two different mechanisms to
fight the extracellular invaders and the intracellular invaders.
It uses proteins called antibodies to fight extracellular invaders,
and it uses a cell-mediated immune response to fight viruses;
the invaders inside cells.
This is a key division in adaptive immunity and are quite important,
for example in using vaccines and developing vaccines.
We need two different sorts of adaptive immune response.
Let's talk first of all about antibody-mediated immunity.
Antibodies are proteins that are produced by
the body and released either into the bloodstream or onto body surfaces.
Now, as we pointed out before,
there's a diverse array of different microbes that are attacking
the body and we therefore need different antibodies for different purposes.
Some antibodies protect the inside of the body,
some antibodies protect the surface of the body.
What they do, is they mark invaders for destruction.
They bind invading organisms,
mark them for destruction.
They only work outside cells,
so only if the organism is outside a cell will antibodies work.
The term we use to describe this sort of immunity is also called humoral immunity,
because antibodies are found in body fluids.