An allergic reaction will not occur on the first exposure to a substance.  The first exposure allows the body to create antibodies and memory lymphocyte cells for the antigen.  However,<unk> s often contain many different substances, including dyes, which could cause allergic reactions.  This can cause an allergic reaction on the first administration of a<unk> .  For example, a person who developed an allergy to a red dye will be allergic to any new<unk>  which contains that red dye.Medications can cause allergic reactions through various mechanisms. The<unk>  can either act as a direct antigenic particle, or it can cause activation of immune cells by the direct interaction with immune cell receptors.
Drug allergy pathophysiology
Allergy pathophysiology
Stevens-Johnson syndrome pathophysiology
Anaphylaxis pathophysiology
Sarcoidosis pathophysiology
Lymphomatoid granulomatosis pathophysiology
Mixed connective tissue disease pathophysiology
Atopic dermatitis pathophysiology