Why is the immune system so fast to respond to allergens, but take days to fight off a legitimate infection?
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The immune system responds quickly because 1) it has memory to that antigen and 2) it has developed an IgE antibody response to the antigen. IgEs behave differently than IgG and IgM antibodies. These antibodies are very long lived and circulate in your system ready to bind their antigen. When they do they engage Fc epsilon receptors on mast cells and basophils, which leads to degranulation and release of histamine. Some times it is such a huge response that it requires an epipen to save a person’s life. BTW this is part of the innate immune response.
For a new infections your immune system creates an adaptive immune response. This takes ~7-10 days to amount a full immune response and create memory T and B cells. Once you have that memory it last a long time and sometimes for life. If you get reinfected by the same bug the memory cells usually mount an efficient response to clear it in such a way that you don’t notice and don’t have any symptoms.
Hope this helps.
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