What happens when you get bitten by a mosquito? You get sick? – No, not instantly, and not always.
When a (female) mosquito bites a human, she sucks out the human blood while also injecting saliva and anti-coagulants. The foreign substance from the mosquito is not venomous. And, the diseases that the mosquito caries is not from the mosquito’s system itself, rather, it comes from three disease-causing organisms: the protozoan Plasmodium parasite (malaria), the parasitic filarial worm or nematodes (human elephantiasis and canine heartworm disease), and the arboviruse agents (yellow fever, dengue, LaCrosse encephalitis, St. Louis encephalitis, Western, Eastern and Venezuelan equine encephalomyelitis, and other viral diseases).
It is these organisms that actually cause the disease and the mosquitoes only act as means to carry them along with the disease, which is why mosquitoes are not called disease-causers but rather disease-carriers.
The only thing that really comes from mosquitoes are the mosquito bite reactions caused by the puncturing of the skin by the mosquitoes proboscis and the introduction of its irritant anti-coagulant saliva leading to succeeding skin reactions like having little red bumps on your skin that are often itchy. These mosquito bite reactions occur very often although the most likely reactions for each individual stung by a mosquito would of course vary widely.
Some people would have little or no mosquito bite reactions, while others can become hyper-sensitive with mosquito bite reactions leading to allergies, blistering, bruising, and extensive inflammatory reactions. For any given individual though, the initial mosquito bite reactions would not be obvious at first. Only subsequently, the bites would cause the body’s immune system to develop antibodies and the bite becomes inflamed and itchy within a 24-hour period. This explains why mosquito bites don't appear instantly but only become increasingly itchy after long hours.
Also most of the time, there are no mosquito bite reactions whatsoever if an adult person gets bitten only once as his body may not find it necessary to release anti-bodies against a single mosquito bite. With more bites, the sensitivity of the human immune system increases, and an itchy red bump appears on the skin after the immune response has broken the blood vessels around the bitten area followed by fluid collecting under the skin. This reaction is common in older children and adults but the usual reactions in young children are more obvious and instant.
So there it is; the most common effect when the skin gets irritated from a mosquito bite is swelling, itching, and stinging, as the body’s immune system reacts against the foreign substance injected by the mosquito. This though can lead to irritation or a serious allergic reaction – face swelling, difficulty in breathing, fever, and even shock.