Why is there a hard lump where a bug bit me?

When a bug bites, its saliva goes into your skin and your body sends histamine to the spot, which is why bites itch and swell (Cleveland Clinic). Firm is one normal way for that reaction to look. A mosquito bite can start as a puffy bump within minutes, then turn into a hard, itchy, reddish-brown bump a day later, or a patch of small blisters, or a dark spot that looks like a bruise (CDC). So a lump that felt soft last night and is firm this morning isn't a sign that something went wrong overnight.

Itching and mild swelling usually clear within a few days, and some bites take a week or two to heal fully (Cleveland Clinic). On brown or black skin you may see very little redness, but the lump is still there to feel, so judge it by touch (NHS).

In an allergic reaction, the swelling is much larger than you'd expect from the bite, sometimes more than 4 inches (10 cm) from end to end. It usually starts within 15 minutes of the bite, though it can begin up to six hours later (Cleveland Clinic). Strong reactions to mosquito bites are likelier in children, in adults bitten by a mosquito species they haven't met before, and in people with immune system disorders (CDC).

Size is only part of it, so watch how the swelling changes over the next day. Swelling that keeps spreading across more skin, or that shows up in other parts of your body besides the bite, is a reason to see a provider. If you've had a big swelling at a bite before, tell the provider, because one milder allergic reaction makes a stronger one more likely later (Cleveland Clinic).

A lump that lingers, or keeps coming back, can be what dermatologists call a persistent insect bite reaction, or papular urticaria. It shows up as clusters of itchy red bumps, each between 0.2 and 2 cm across, with a tiny dot in the middle where the bite went in, mostly on the legs and other uncovered skin. The clue is the timing: new bumps form just as old ones start to clear, and a fresh bite can bring old ones back (DermNet). Each spot stays for days to weeks, so the whole pattern can run for months (Australasian College of Dermatologists).

Some of these lumps blister one to three days after they appear (Australasian College of Dermatologists). The reaction is more common in children, who haven't built up tolerance, and in adults who've traveled somewhere new. It settles over months or even years as the body gets used to the bites, and the bite that started the pattern is rarely noticed (DermNet).

Fleas and mites that live on cats and dogs are the most common cause, although plenty of people with it have no pets, and it can be hard to tell what a person is reacting to (DermNet). If you have a cat or dog, part the fur and look for fleas on the skin around the ears, neck, back, and belly (Cleveland Clinic). Whether your lumps fit this pattern is a question for a clinician, who usually decides by looking at the skin and the grouping of lumps, and sometimes takes a small skin sample (Australasian College of Dermatologists). If your lump didn't appear until days after the bite, the delayed reaction page explains why that happens.

A lump that hurts, feels warm, and has a softer middle can be a skin abscess, a pocket of pus. It starts when bacteria get in through a cut or along a hair follicle (NHS), and a bite you've scratched open is a cut.

Dermatologists say never to squeeze or pierce a boil, a pus-filled infection around a hair follicle, because that can spread the infection. Their home advice for a boil is a warm compress held on for 10 to 15 minutes, three or four times a day (American Academy of Dermatology). Ask a clinician or pharmacist whether that suits your lump before you try it. The UK's National Health Service (NHS) says to call for an urgent appointment if the lump is painful, red, or hot, if you feel hot, cold, or shivery, or if swelling or redness spreads away from it (NHS). The infected bug bite guide goes through pus, crusts, and red streaks in more detail.

Whatever started it, the NHS says to see a doctor if a lump is getting bigger, if it is hard and will not move when you press it, or if it stays and does not go away, on any part of your body. A doctor can check what's causing the lump (NHS).

Press the lump, then try to slide it a little under your fingertip. If it feels fixed in place, book a visit even when you're sure a bug started it. No source here gives one number of days for an ordinary bite to soften. For a boil, dermatologists say you should make an appointment if the swelling or pain goes on for several days, or the lump doesn't go away (American Academy of Dermatology). They don't name an exact day, and that is the closest limit these pages give for a bite lump too.

Below the knees, groups of bites point toward fleas, and mite bites make very itchy lumps that sometimes blister. Horsefly bites are large and painful, may bleed, and often get infected (NHS), so if you think a big, tender lump came from a horsefly, have a clinician look at it sooner than you would a small itchy lump.

A bite or sting close to your eye is on Cleveland Clinic's list for contacting a provider, along with open wounds from scratching and symptoms that are worse or haven't started to fade after a few days (Cleveland Clinic). For a baby under one year old, the NHS says any bite or sting should be seen by a doctor. For anyone older, a pharmacist can look at a bite that's hot, red, painful, swollen, or has pus (NHS).

Do these tonight so tomorrow you can tell whether the lump changed.

• Measure across the widest part of the lump with a ruler, and write down the number, the date, and the time.
• Measure again at the same time tomorrow. The same size or smaller is a good sign, and a bigger lump is a reason to call a clinician.
• Look around the lump for newer bumps. A second crop appearing while the first fades is the pattern described above for a persistent reaction.
• If it's on an arm or leg, raise it, and hold a cloth dampened with cold water or ice on it for 10 to 20 minutes to ease pain and swelling (Mayo Clinic suggests this for a mild reaction).

Get care now, and call 911 or your local emergency number, if you have trouble breathing, swelling of the lips, face, eyelids, or throat, dizziness or fainting, hives, or nausea and vomiting. Any one of those is enough to call (Mayo Clinic). Short of that, see a doctor or pharmacist if the swelling gets worse, the site shows signs of infection, or you don't feel well.

A clinician will ask when you were bitten, how many bites there were, how soon the symptoms began, what you've tried so far, and how your skin reacted to past bites (Cleveland Clinic). Write those answers next to the measurement from tonight.

If you can't tell a lump that will soften from one a clinician should see, photograph it, or the insect if you still have it, in the BiteSight iPhone app. If you don't have an iPhone, the same photo tool works on the web. There's no Android app yet.