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DIRECTIONS: Choose the one best response to each question.

A 55-year-old woman who was previously incarcerated has an area of induration that measures 15 mm 72 hours after a skin test with tuberculin purified protein derivative (PPD). A positive PPD skin test for Mycobacterium tuberculosis represents which type of immune reaction?

A. Cytotoxic reaction of antibody

B. Delayed-type hypersensitivity reaction

C. Immediate-type hypersensitivity reaction

D. Immune complex formation

The answer is B. (Chap. 342) Inflammatory reactions initiated by mononuclear leukocytes and not by antibody alone have been termed delayed-type hypersensitivity reactions. The term delayed has been used to contrast a cellular response that appears 48–72 hours after antigen exposure with an immediate hypersensitivity response generally seen within 12 hours of antigen challenge and initiated by basophil mediator release or preformed antibody. In patients previously infected with Mycobacterium tuberculosis, intradermal placement of tuberculin purified protein derivative as a skin test challenge results in an indurated area of skin in 48–72 hours, reflecting a delayed type hypersensitivity reaction by leukocytes. Immune complex formation refers to antigen, complement and antibody coming together to clear antigen, but can also lead to tissue damage when not cleared by the reticuloendothelial system and deposited in tissues like the blood vessels or glomeruli as can occur in some vasculitis syndromes. Cytotoxic reactions to antibody occur when antibodies against cells or tissues bind complement and lead to cell lysis or tissue injury, as occurs in transfusion reactions.

All of the following are key features of the innate immune system EXCEPT:

A. Exclusively a feature of vertebrate animals

B. Important cells include macrophages and natural killer lymphocytes

C. Nonrecognition of benign foreign molecules or microbes

D. Recognition by germline-encoded host molecules

E. Recognition of key microbe virulence factors but no recognition of self-molecules

The answer is A. (Chap. 342) The innate immune system is phylogenetically the oldest form of immunologic defense system, inherited from invertebrates. This defense system uses germline-encoded proteins to recognize pathogen-associated molecular patterns. Cells of the innate immune system include macrophages, dendritic cells, and natural killer lymphocytes. The critical components of the innate immune system include recognition by germline-encoded host molecules and recognition of key microbe virulence factors, but there is no recognition of self-molecules and of benign foreign molecules or microbes. Adaptive immunity is found only in vertebrate animals and is based on the generation of antigen receptors on T and B lymphocytes by ...

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