IMMUNOLOGY

Detailed explanation-1: -IgE is not believed to play an important role in defense against bacterial infections, since it does not activate complement or participate in opsonization [5]. IgE plays a key role in the pathogenesis of allergic diseases, especially mast cell/basophil activation, and in antigen presentation.

Detailed explanation-2: -The complement system is a key part of the innate immune system and plays an important role in the clearance of pathogens and apoptotic cells upon its activation. It is well known that both IgG and IgM can activate complement via the classical pathway by binding of C1q to the Fc regions of these immunoglobulins.

Detailed explanation-3: -Complement activation does not occur in systemic anaphylaxis because IgE immune complexes do not activate complement. In this immediate hypersensitivity reaction, the antigen binds to an IgE antibody on mast cells or basophils, causing release of mediators that produce life-threatening symptoms.

Detailed explanation-4: -IgM is specialized to activate complement efficiently upon binding antigen. IgG antibodies are usually of higher affinity and are found in blood and in extracellular fluid, where they can neutralize toxins, viruses, and bacteria, opsonize them for phagocytosis, and activate the complement system.