Which antibody class binds mast cells through its Fc region and sits at the lowest concentration in blood?

1. IgE is the antibody class usually associated with allergies and anaphylaxis. 2. Its Fc region binds strongly to an IgE-specific Fc receptor on the surfaces of mast cells, so little of it stays free. 3. That is why IgE is present in the lowest concentration in the blood of the five classes. 4. Bound IgE makes mast cell degranulation very specific, so a person allergic to peanuts carries peanut-specific IgE on those cells. 5. IgG is the main blood antibody of secondary responses, which puts it at the opposite end of the concentration scale. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 21 "The Lymphatic and Immune System", section 21.4 The Adaptive Immune Response: B-lymphocytes and Antibodies_