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I’m going to try to make all this science stuff understandable, so please let me know how well I succeed. It took me an entire semester of vet school to understand all this. I promise not to go into that much detail, unless you really want to (please say no LOL). It’s not that important for this discussion anyway. Oh, and this is *just* about adaptive immunity. There’s a whole separate part of the immune system called the innate system that deals with disease in a non-specific manner, but we don't need to know about that right now.

Let’s start with some definitions:

Antigen- anything that makes the immune system react. This could be a virus, a bacteria, or something that should be harmless like peanut butter or pollen.

Pathogen- any antigen that is actually disease-causing (not peanut butter, that's a hypersensitivity reaction, not a disease)

Antibody- a protein that attaches to an antigen to make the antigen ineffective, or that targets the antigen for destruction by other cells and proteins.

B-Lymphocyte- a type of white blood cell that produces antibodies

T-Lymphocyte- a type of white blood cell that destroys infected cells (cytotoxic T-cell)

Antigen Presenting Cells (APCs)- cells that engulf a pathogen, process it, and shows the antigen to lymphocytes

Passive immunity- maternal antibodies, passed along from the mother to the baby through the placenta and in milk.

Active immunity- the antibodies that the individual makes themselves by being exposed to an antigen

Memory cells- B lymphocytes that can quickly produce antibodies, or T lymphocytes which can quickly recognize and destroy infected cells in the body

Vaccination- a substance that is enough like an actual pathogen to trick the immune system into targeting the actual pathogen if it is ever seen.

Adjuvant- a substance added to a vaccine in order to stimulate a better immune response.

Sterilizing immunity- when a vaccine prevents an animal from showing any signs of disease and keeps the animal from being contagious to others

Non-sterilizing immunity- when a vaccine dramatically reduces the clinical signs and duration of illness for a disease, but the animal can be contagious to others

What happens when a naive animal is exposed to an antigen

The first time an unvaccinated animal is ever exposed to a disease, a lot of things happen so that if the animal survives the infection (kind of a key point), it will be better protected next time.

First, the pathogen is engulfed by antigen presenting cells (APCs) which are pretty much garbage collectors and sifters. APCs just collect whatever is foreign to the body, break down and sort that foreign material, and if something is scary, the APC flags the antigen for further processing by lymphocytes (B cells and T cells).

B cells recognize the antigen, then turn into plasma cells and secrete specific antibodies, the substances that bind to antigens and inactivate them. Antibodies are kind of flaggers that invite other cells and substances to destroy them along with the pathogen. B cells also make memory B cells, which are ready to produce antibodies as soon as the same antigen is seen again. Memory B cells allow you skip all that APC stuff and waiting for the B cell to turn into a plasma cell and then finally secrete antibodies. Memory cells are ready to go.

Cytotoxic T cells recognize the antigen and turn into active cytotoxic T cells. They find other cells in the body that are infected with the pathogen and destroy those cells. Cytotoxic T cells also make memory cytotoxic T cells, which are always searching for infected cells to destroy.

The second time an animal is exposed to the same pathogen, all those things happen again, but there is already a small population of memory B and cytotoxic T cells ready to become active and start working. But something else also happens- those helper T cells produce memory helper T cells, which produce even more memory B cells and more memory cytotoxic T cells. Which means the second time an animal is exposed to the same pathogen, the immune response is faster and better. This is why most vaccines need a series of at least 2 vaccinations to be fully protective.

How are animals protected from disease?

If a puppy or kitten is born to a bitch or queen that has been vaccinated or who has been exposed to a disease, then the mother can provide passive immunity to the offspring. These are maternal antibodies, passed through the womb and/or milk. Maternal antibodies are ready to fight disease right away, just like antibodies produced by the individual. Unfortunately, maternal antibodies only last 8-12 weeks, which means at some point, the puppy or kitten is going to be susceptible to some pretty horrific diseases such canine distemper or parvo, or feline calicivirus or panleukopenia. We can’t have that! Even though these diseases are potentially survivable, it is NOT CHEAP to hospitalize pets for days on end getting IV fluids and IV antibiotics, and other treatments. Plus, these little babies are SICK. And there is absolutely no need for any animal to have to suffer this way because we have vaccines!

How Vaccines Work

Vaccines are basically either little parts of a pathogen or an inactivated (killed or attenuated) pathogen that trigger the exact same immune response without actually causing disease. There are essentially 3 types of vaccines: modified live vaccines (MLV), killed vaccines, and recombinant vaccines.

Modified live vaccines are exactly what it sounds like- vaccines made from the actual pathogen, but changed in such a way that they don’t cause disease. Because the MLV is so close to the actual disease, they are excellent vaccines, stimulating a massive immune response. They typically last a lot longer than other types of vaccines as well.

Killed vaccines are also exactly what they sound like- the pathogen is treated typically to heat, enough to kill the pathogen but keeping the proteins intact. Injecting a bunch of dead pathogen will not stimulate the immune system though, so substances called adjuvants are added to killed vaccines in order to make the immune system recognize the pathogen and react. These vaccines typically don’t last as long as MLV, because they don’t replicate in the body. Furthermore, in cats, the adjuvants can cause a very aggressive type of cancer called a vaccine associated sarcoma. Thankfully, there are other options available for cats so they never have to receive an adjuvanted vaccine. Unfortunately these vaccines are more expensive than the killed ones and some practices are still risking their patient’s lives by administering these vaccines. All of our feline vaccines are non-adjuvanted, always have been, and always will be.

Recombinant vaccines are pretty slick. They take the part of the pathogen that the immune system recognizes and splice it into a harmless live virus, usually canarypox, that can replicate inside the body. So immune system reacts to these vaccines just like a MLV, with usually long-lasting immunity.

Why puppies and kittens need a vaccine series

If vaccines are so great at preventing disease, why do puppies and kittens need so darn many of them? The answer surprisingly is not that their immune system doesn’t work that well because it's immature. It works just as well as an adult animal. The problem is those darn maternal antibodies. Remember those? Well, turns out that maternal antibodies are so effective at destroying antigens, they destroy our vaccines too! I’ll be honest with you, most of the DAPP and FVRCP vaccines we administer are going to be destroyed and are useless to the puppy or kitten.

The problem is that we never know exactly when those maternal antibodies are going to wear off in each individual animal. It could be 8 weeks, it could be 12 weeks, and it can vary even between litter mates. And if the animal doesn’t have maternal antibodies and it isn’t vaccinated, it’s going to be susceptible to disease. Alrighty then, we’ll just vaccinate the puppy or kitten every 3 weeks until such an age that we KNOW there will be no more maternal antibodies to destroy our vaccine. There is a very small window of susceptibility as shown in this graph, where the maternal antibodies can destroy the vaccine, but they aren’t enough to protect the pet. Thankfully that is a small window, and with vaccines every 3 weeks, it is unlikely that the pet will get sick. However it does show just how important it is to keep with the vaccine schedule.

We have to REALLY stimulate the immune system to make sure that the puppy or kitten has enough memory B and T cells to be ready to secrete antibodies as soon as the animal is exposed to the disease. Because we don’t even want them to get sick at all. That’s why we have to administer 2 vaccines that “count.” If our vaccines are being destroyed by maternal antibodies until 12 weeks of age, then only our 15 and 18 week vaccines are actually protective. But that’s plenty. It’s also the reason we don’t go back and administer a full 5-vaccine series to an older animal. They don’t have maternal antibodies so all of our vaccines are effective and protective. Yay vaccines!

We’re going to talk about over-vaccination next week. In the meantime, ask questions and let us know what you want to talk about next!