Overview

The purpose of the immune system is to help your body fight off foreign invaders.

Immunity

The body's ability to find off alien invaders.

Pathogens

The alien invaders.

They can be: bacteria, viruses, fungi, protozoa, prions.

Antigen

A specific part of the pathogen (usually a protein).

This is the part that is recognized by the antibodies.

Active Immunity

Your body creates an army of soldiers to fight off a specific type of alien invader.

This can happen naturally by being infected.

It can also happen artificially by being given an immunization.

An immunization is an inactive version of a given infectious agent.

When you create these soliders, you also store the plans for them in case you are infected again.

(see "Natural" and "Artficial" in the diagram above).

Passive Immunity

You can "rent" an army of soliders to fight of a specific type of alien invader by getting them from someone else who is already producing those soldiers.

It will help you fight off the alien invader this time, but will not give you the plans for the soliders for the next time.

(see "Antibody Transfer" in diagram above)

Antibodies

These are the soldiers in the army that fights off the alien invaders.

They are specific to one type of foreign invader.

See the diagram below for how antibodies "fit" antigens. (Think about enzymes and substrates)