How is antiserum different from vaccination?
An immunization is a challenge to the immune system that looks to it like the target pathogen - often with an adjuvant to do enough minor mischief to convince the immune system that this is a really bad guy that needs a SWAT team response. It might be made out of:
- pieces of killed pathogen,
- pieces of killed related pathogen,
- engineered molecules similar to a target site on the ,
- live related pathogen (enough like the bad guy to provoke a cross-reaction to the bad guy, but not enough like the bad guy to cause the disease),
- live attenuated pathogen (an artificially weakened version of the actual disease - essentially an engineered "live related pathogen"
- the actual, full-bore, pathogen itself - but administered in a way that leads to a less severe (i.e. survivable) case of the disease,
The immune system has an enormous number of small clones (just a handful of cells) that each produce a different antibody (and can produce one or more of several types of response against a pathogen), and essentially any that produce antibodies against the body itself have already been killed off. When the body signals "I'm being attacked", by either a disease or an imunization (which mimic a disease) those that recognize the antigen go into rapid reproduction, and a fraction differentiate into active forms. This takes about three days - but after that you have a LARGE number of mature immune cells that attack that pathogen, along with a boosted number of not-yet-matured "memory cells". This doesn't stop an original infection. But it cleans up after it, and blocks (or mitigates) future infections by attacking the pathogen as soon as it shows up. If you get another case of the disease - or a booster shot - the memory cells will repeat the process, making the immunity much stronger.
Vaccination is a particular case of a "live related pathogen": One of the (closely related) cowpox or vaccinia viruses, somewhat more distant relatives of smallpox, is used to create a minor infection (generally one scarring pimple, unless you scratch and spread it). This activates the person's immune system against both the vaccine's virus and its relatives, including smallpox.
The Sabin "live virus" polio vaccine works the same way, using a weaker, mutated, version of the polio virus. (Its predecessor, the Salk vaccine, uses the outer coat of killed polio virus.) A live virus actually produces a disease process lasting several days, until the immune system clears it up. This creates a stronger and longer lasting immunity than a simple challenge with dead virus pieces. (It also is contagious: Some people who weren't administered the immunization "catch" the "fake disease" from those who recently were immunized.)
Inoculation consists of deliberately administering the pathogen. In the case of a disease, it means causing the disease, in a way that can be treated or is otherwise is survivable, leaving the recovered person immune. Before vaccination, inoculation was used for Smallpox. If you catch smallpox by inhaling the virus, you're likely to have a severe case, either dying or maybe being horribly scarred. If you catch it by having some pus from an infected person get into a cut in your skin, you're likely to have a mild case, with only localized scarring, and then (as a survivor) be immune. (Unfortunately, while you have the case, you're infectious with the disease, so others can catch it the bad way. That's the biggest reason that vaccination was such a drastic improvement.)
Unfortunately, immunizations are usually too late to protect you against a disease you already have. (A notable exception is rabies, which works its way slowly up the nerves to the brain, giving time to immunize before it becomes acute, incurable, and fatal.) An antiserum works immediately.
One of the ways the immune system attacks pathogens is for the antibody-producing cells to shed the antibodies into the blood and lymph. These antibodies then attach to pathogens, doing things like blocking the active sites, sticking them together into clumps, or otherwise marking them for attack by other immune--system cells.
An antiserum is a big dose of extracted antibodies against the pathogen. It gives you most of the advantages of being immunized (other than producing the antibodies yourself), but RIGHT NOW. So it can be used against a disease that is already in progress.