When you get a shot, like a flu or COVID vaccination for example, they typically put it in your upper arm or your thigh. Are they just sort of injecting it into the muscle? Or into a vein? Either way, where does it *go*?
I have seen videos of when people get things like local anesthetic where the skin starts to balloon out where they put the liquid. Does that sort of thing happen just deeper in the body? I know our body is mostly liquid but does it just absorb? Maybe redistributed ?
In: Biology
The vaccines you mentioned are injected directly into the muscle. Your muscles already have a bunch of water in them so the water that gets injected is handled with that system.
For the actual vaccine part: your body recognizes that a foreign substance is in you and sends immune cells to fight it/break it down. At first, it’s basically throwing a bunch of random antibodies at it hoping it works. Eventually, your body figures out what works and makes more of those antibodies and retains memory of the infection.
Here’s a paper from Moderna in 2019 if you want to see a small slice of how they developed their delivery platform: https://www.cell.com/molecular-therapy-family/nucleic-acids/fulltext/S2162-2531(19)30017-4
For injections into a vein, it just goes directly into your bloodstream, but that’s not typically how vaccines are given. Vaccines are mostly given into muscle tissue, where the liquid just spreads out in the tissue and eventually is absorbed into the bloodstream. Some injections are also given subcutaneously (into the layer of fat just below the skin) where it forms a small bubble of liquid that’s absorbed into the bloodstream.
There are different kinds of injection. The main four types are:
* intramuscular (=into muscle tissue), needle at a 90 degree angle to the skin
* subcutaneous (= under skin but above muscle, aka into fat cells), needle goes in at about a 45 degree angle
* intravenous (=into a vein; arteries aren’t used for this purpose because it would be unsafe), needle goes in at about a 25 degree angle
* intradermal (=within the layers of your skin), needle goes in at roughly a 10-15 degree angle
Different medicines call for different kinds of injections. A vaccine often doesn’t really matter WHERE it ends up, just as long as it gets your body’s attention, so the simple intramuscular injection is perfectly fine. Most medicines or IVs (e.g. blood, glucose solution, saline, etc.) need to be introduced directly into the bloodstream, so those have to be intravenous.
I was a bone marrow donor for someone last year, and for that I self-administered injections of a compound that makes your bone marrow grow, so that it would go into my bloodstream. Those needed to be subcutaneous, as this results in a slower, steadier rate of the drug entering the body and traveling all over.
To answer your last few questions: That “balloon” effect most likely means an intradermal injection, which generally results in very fast absorption by the body, which is useful for pain medication, especially if it’s injected nearby the source of the pain. And yes, the body really just just absorb and redistribute the liquid. Once it’s in your circulatory system, it spreads out, just like if you’d put dye in a collection of glass pipes with flowing water, the dye will spread out until the system is mostly uniformly colored. Depending on where you put the dye, it may spread quickly or slowly–that’s exactly what doctors are trying to make use of when they use different types of injection.
The body will (eventually) absorb the injected medicine into the bloodstream. The medicine passes into the network of blood vessels and is circulated in the body.
There are different types of injections, for different purposes. In general, how an injection is delivered impacts how fast the medicine is absorbed into the bloodstream — and thus the rate at which its effects are felt.
1. Injection directly into bloodstream (vein) — eg; certain antibiotics
2. Injection into deep muscle area — eg; tetanus shot
3. Injection under the skin layer — eg; insulin
4. Injection just under the top layer of skin — eg; testing for tuberculosis
These are listed from fastest to slowest.
There are other kinds of injections for more specialized purposes / reasons, but these are some common ones.
The other comments are correct about the eventual immune response to substances injected into your body, but I just wanted to touch on this one point: one of the main reasons why vaccines are intramuscular injections is simply ease of use. It’s a lot harder to get into a vein consistently, on every single person receiving a dose, than it is to get into the meaty part of a fairly large muscle. Phlebotomy is a particular skill that needs practice to do properly, and even then there are confounding factors (small veins, rubbery veins, rolling veins). Even the pros who draw blood from hundreds of sick, dehydrated people in a hospital won’t get it right on every attempt. Why bother expecting your pharmacist to master this skill and potentially mess up an intravenous injection when they could do something that’s faster, easier, and still works just as well?
Those vaccines go into the muscle tissue, they contain deactivated viruses or viral rna that will trigger an immune response from your body’s immune system. in this way your body learns to produce antibodies against the active form of the same virus, so if you are ever exposed to the active virus your body will be able to fight it off before you get sick.
Your immune system responds to vaccines by recruiting immune cells to the injection site that pick up antigen, and then go to the nearest draining lymph node, which is basically a factory for antibody production.
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