Question 222

Question:

A healthy 19-year-old man receives a tetanus immunization booster prior to induction into the US Marines. Six hours later, he has pain and massive swelling at the site of injection. The following day, the skin breaks down, forming an ulcer at the site. Which of the following events plays a critical role in this reaction?

Correct Answer: C.  Local fixation of complement by preformed circulating antibodies

This patient is having an immune complex (type III) hypersensitivity reaction to the tetanus booster commonly referred to as an Arthus reaction. Because the patient has been previously immunized, preformed circulating antibodies locally fixate complement at the injection site due to the high load of tetanus toxoid in the booster. This results in a local inflammatory vasculitis with pain, edema and skin ulceration.
Accumulation of mononuclear cells occurs in cell-mediated (type IV) hypersensitivity. Langerhans cells are dendritic cells of the skin that present foreign antigens in response to infection or foreign bodies, but antigen capture in the epidermis has no role in the Arthus reaction. Histamine release is a key component of immediate (type I) hypersensitivity. IgM and IgG proliferation occur in antibody-mediated (type II) hypersensitivity reactions.