Active/passive acquired immunity

[Snorlax]

Could someone provide a definition for the two, please?
Would be appreciated, thanks.

[simpak]

Active immunity is where the immune cells of the body actually participate in generating an immune response to protect a person against infection with a pathogen: for instance, that person's T cells and B cells are activated and the person's cells synthesise antibodies 'actively' themselves.  The host cells are playing an active role in the process of creating the immunity.  It is permanent because induces memory.  It can come about either through direct encounter with the pathogen or via vaccination with a component of the pathogen - both of these will induce a B and T cell response against the antigen and (usually) memory cells will be created.

Passive immunity involves making someone become immune to a pathogen without their cells needing to actively do anything to contribute to the immunity.  So you might give someone pre-made antibodies intravenously and those antibodies will provide immunity against the agent the antibody is specific for without the requirement for processing of the pathogen and activation of  host B cells and T cells.  Alternatively, the person might acquire those antibodies from their mother whilst in the womb (some antibodies can cross the placenta).  Both of these forms of immunity are temporary because the host immune system never actively processed the antigen and actively produced its own antibodies against it, so you don't get any memory cells to continuously survey the body for the antigen and to produce antibodies for a long time afterwards.  The antibodies you provide during passive immunity will degrade/deplete over time and eventually you won't have any left and if the person were to re-encounter the antigen they would get sick because there is no long term memory response to detect them.

Hope that helps - it's more of an explanation than a neat definition but I'm sure you can summarise.

Edit: I should also say that active immunity takes some time to develop (a few days for the antibodies to be generated and then memory cells take a while to develop properly) whereas passive immunity is immediate because the body doesn't have to do anything and you've provided the antibodies for it.

[Snorlax]

Yeh thanks! Much greater summary than my textbook. Appreciate it.