Immunization in Spybot S&D

[Alpha Force]

What exactly does it do?

[screw like bunnies]

Immunize Search & Destroy
Beginning from version 1.2, Spybot-S&D allows you to immunize your computer against some spyware. It currently offers three different immunities:

Permanent Internet Explorer immunity

Similar to JavaCools SpywareBlaster, this allows you to tweak some internal Internet Explorer settings to block the installation of known spyware (and similar threats) installers. Spybot-S&D is able to set all entries for those that are in its database to be blocked. If you want to distinguish, you should install SpywareBlaster.

Permanently running bad download blocker for Internet Explorer

This is a second layer of protection for IE. While the Permanent Immunity blocks installers by their ActiveX ID, this one blocks anything that should come through by different aspects.
You can view a log of blocked installers in the Tools / Resident section.

Permanent Opera immunity

This list shows all Opera profiles, and how many of the plugins that Spybot-S&D knows as bad are already blocked in each profile. To set the complete protection, just tick the checkbox in front of the profile you want to protect. To remove the protection, untick the checkbox.

Not many browser plugins are currently working for Opera, but the try to install some may crash Opera, and ActiveX support for Opera is available in third-party beta software, so use this function as a pre-caution against future misuse of Opera through spyware.

!This feature will (certainly) only be displayed if you have Opera installed on your system!

[Manhattan Project]

It provides protection by not allowing known spyware/adware to be installed in the system.

[elijah_harper]

Despite this, immunisation is not infallible- you should still do a weekly scan for spyware. This is because it blocks around 1000 types of spyware, and yet at least 22000 (!) exist. Still handy though.