I think some scientists are confusing the anaphylaxis resulting from activation of mast cells with the activation of the complement system which can look like anaphylaxis. With mast cells, the antibodies that result from an exposure to a foreign toxin or protein are attached to the mast cell FIRST. Then when a subsequent exposure to the same toxin or protein occurs, the toxin or protein will attach to the antibodies that are already attached to the mast cells. This causes a violent release of histamines that will result in an immediate anaphylactic reaction that can be almost immediately relieved by antihistamines. Yes, this can happen immediately after a covid shot if the person is sensitized to polyethylene glycol or any other foreign substance that may be a contaminant.

Normally when antibodies are produced from exposures to foreign proteins that are injected by needles, these antibodies do not attach to mast cells or any other immune cells. These antibodies can take two weeks or more to develop (21 days) and will attach to the foreign protein on a second exposure. After that, the antibodies will attach to immune cells. They are not attached to immune cells first, as in a classic anaphylactic reaction. In some subsequent exposures to the same toxin or proteins, depending on how many antibodies are immediately available, the antibodies will attach to the toxin or protein in such a way that it signals the complement system to become activated. In some cases, this can be extremely violent and inflammatory, looking like anaphylaxis.

This would explain why a 21-day window or longer is required. The antibodies must be present, and it can take that long to produce them. It also explains why subsequent exposure to some crystalline substances will not do this as they generally do not cause antibody production.