Final answer:
The IgE class of immunoglobulins is responsible for responding to allergic reactions and parasitic infections by binding to mast cells and mediating immune responses.
Step-by-step explanation:
The class of immunoglobulins that is usually formed in response to allergic reactions and parasitic infections is IgE.
This antibody binds to mast cells and is involved in causing antigen-specific degranulation during an allergic response. On top of its role in allergies, IgE also mediates the immune response to parasitic worms, making it crucial in the body's defense against these types of infections.