What happens if you don't remove a splinter or foreign body?

As with any medical issue, there are many variations to this answer. Basically, an unremoved splinter or foreign body can lead to:

1. Persistent irritation either mechanical or chemical

2. Become walled off by the body and become essentially inert

3. Become walled off by the body but due to size or location, the walled-off splinter still is a mechanical irritant

4. An infection

5. Be "spit" out of the body

6. Any combination or permutation of the above

Factors that influence the outcome of the splinter:

1. What is the material? Wood, treated wood, metal, glass, painted wood, plant thorn, etc.

2. What is the size relative to the location?

3. Where is the splinter relative to depth and anatomic part?

4. How dirty was the entry wound or the skin i.e. was it a farm contaminated injury or did it occur in a "cleaner" dry setting?

5. Is the person immunocompromised?

6. Does the person have colonization with MRSA or some other type of iatrogenic induced resistant bacteria?

These are some common examples:

Material

Treated wood contains a variety of chemicals from water-borne chemicals to creosote to oil. Treated wood, for example, is used in decks with one class of more mild chemical or in telephone poles with oil-based products. Treated wood often gives a persistent inflammatory reaction that is an irritant until the wood is removed and the part is washed out and irrigated out fully.

Depth and location

Metal splinters also called slivers, occur often in the hands of machinists and those who fashion metal parts. They often lay inert for years unrecognized as the metal does not interact significantly with the body. If the sliver is superficial it often spits. If it is deep enough to cause irritation to the superficial nerves (in the superficial fascial layers) then often treatment is sought out as it is a painful mechanical irritant. If it goes deeper, below the subcutaneous fatty layers, it can sit for year and years without being noticed, and years later found on an X-ray incidentally.

Location and "walling off" and "spitting"

A small glass shard or a small splinter of plain old inert regular living wood on the bottom of the foot may cause irritation for a few days. But let's say you can't get to it and to remove it hurts. You soak your foot but it just isn't there. After a few days of walking around you start to disregard it but it's still there. Weeks later you will likely have a small bump. Your body has walled it off but the bump is so small that it just sits in the calloused part of your sole of your foot and that's it. Once and a while you notice it, then one day it's gone.

Either it has spit with the growing skin i.e. it was only as deep as the dermis and as the skin grew out it grew out too, as the dermis became epidermis and hardened the skin and poof it's gone. Or it is walled off in an area that you never notice or care to bother about again.

Walled off but not gone and irritating

If the location is just right and/or some living skin cells are located deep in the tissues with the splinter, then an inclusion cyst or an epidermal inclusion cyst will grow layer and layer added to the foreign body or splinter. Finally, what was a little inert splinter is now a hard lump that grows and grows (think cultured pearl).

Finally, the hard lump itself is annoying and removal entails not only the offending foreign body but the cyst or mass of tissue itself. Sometimes, these happen even in the absence of a splinter, but when the skin is inoculated by the puncture small bits of living epidermal cells literally "tattoo" the deeper tissues and continues amorphic growth from keratin or skin proteins. This gives a hard cyst filled with a white pasty but smooth proteinaceous material (keratin).

The other factors listed are important, but for most of us, the above is what we deal with. However, **infection** is the chief of mischief, the one bad actor that throws all else off.

**Infection**

Any puncture wound or splinter can result in an infection, although certain types of material are associated with specific types of bacteria, fungus or species of microorganism.

Rose thorns: Sporotrichosis

Fish hooks: Mycobacterium marinum

Tetanus: any puncture but soil-borne organisms are usually responsible

Pseudomonas: a puncture wound through a sneaker or the sole of a foot

Staphylococcus species colonize our bodies and environment. The staphylococcus species of bacteria are everywhere. MRSA is a methicillin resistant staph that requires special antibiotics. Any splinter can cause a bacterial infection, and there are others.

Infection is the exception for all of this, INFECTION must be treated and concurrent removal of the splinter is done too as the foreign object can be an ongoing source. 

As far as the *sort of* urban myth of lead poisoning or toxicity or the dreaded "blood poisoning" from the perennial pencil point, or something else, usually it is a story that related to infection or some other issue that complicates this process. 

The short answer is "it depends..."