Questions: Type II Hypersensitivity: Antibody-Mediated Cytotoxic Reactions

Question 1

A patient develops IgG autoantibodies against their own red blood cells. A clinician proposes that complement-targeted therapy is unnecessary because 'the complement system can handle everything.' What is wrong with this reasoning?

• A. Complement always acts too slowly to destroy RBCs, so complement inhibition would have no therapeutic benefit
• B. Cell destruction can also proceed through ADCC by NK cells and ADCP by macrophages via Fcγ receptors, completely independent of complement — blocking complement alone would leave these pathways intact
• C. Red blood cells cannot be targeted by antibody-mediated mechanisms because they lack nuclei
• D. ADCC requires CD8+ T cells and is not relevant to antibody-coated red blood cells

Answer: (B)

Type II hypersensitivity has three distinct destruction mechanisms: complement-mediated lysis (MAC formation), ADCC (NK cells and macrophages using Fcγ receptors to kill antibody-coated cells without complement), and ADCP (macrophage phagocytosis of opsonized cells via Fcγ receptors). All three can operate independently. A common misconception is that complement is required — in fact, ADCC and ADCP proceed entirely without complement, which is why complement inhibition alone may not adequately control autoimmune hemolytic anemia.

Question 2

Graves' disease is classified as Type II hypersensitivity, yet it results in thyroid hyperfunction rather than thyroid cell destruction. How is this consistent with Type II mechanisms?

• A. Graves' disease is actually misclassified — it belongs to Type III hypersensitivity because the antibodies form immune complexes
• B. The antibodies in Graves' disease bind the TSH receptor and mimic TSH signaling, stimulating the receptor rather than targeting the cell for destruction — demonstrating that cell-surface-targeted antibodies can activate or block receptor function, not only lyse cells
• C. Complement is absent from thyroid tissue, so ADCC and ADCP cannot proceed, leaving cells intact but hyperactivated
• D. IgA antibodies, which cannot activate complement or ADCC, are responsible for Graves' disease

Answer: (B)

Graves' disease illustrates that Type II hypersensitivity is not synonymous with cell killing. When antibodies bind a cell-surface receptor, three outcomes are possible: (1) complement/ADCC/ADCP destroy the cell, (2) the antibody blocks receptor function (as in myasthenia gravis, where anti-AChR antibodies block neurotransmitter binding), or (3) the antibody activates receptor signaling (as in Graves', where anti-TSH receptor antibodies constitutively stimulate thyroid hormone production). All three are Type II because the antigen is fixed to a cell surface — the classification reflects antigen location, not destruction outcome.

Question 3

The tissue specificity of Type II hypersensitivity diseases — why some affect red blood cells, others the thyroid, others the neuromuscular junction — is determined by which cell-surface antigen the pathogenic antibody recognizes.

• T. True
• F. False

Answer: True

Unlike Type III hypersensitivity, where immune complexes deposit wherever blood flows (especially in vessel walls and glomeruli), Type II disease is targeted to whatever cell or tissue displays the antigen the antibody recognizes. Hemolytic anemia occurs because anti-RBC antibodies are specific for red blood cell surface antigens; Graves' disease affects only the thyroid because the TSH receptor is thyroid-specific. Organ specificity is a direct consequence of antigen specificity.

Question 4

Type II hypersensitivity reactions usually result in destruction of the target cell, making them uniformly cytotoxic.

• T. True
• F. False

Answer: False

Type II reactions range from cell destruction (hemolytic anemia, hemolytic transfusion reactions) to receptor activation without destruction (Graves' disease — thyroid hyperstimulation) to receptor blockade without destruction (myasthenia gravis — impaired neuromuscular transmission). The common thread is antibody binding to a cell-surface or matrix-bound antigen, not the outcome. The clinical picture depends entirely on what the antibody binds and what happens when it does.

Question 5

What distinguishes Type II from Type III hypersensitivity in terms of antigen location, and why does this distinction explain the pattern of tissue damage in each?

Model answer: In Type II, the antigen is fixed — bound to a cell surface or extracellular matrix. Antibodies bind and direct damage precisely to the cells or tissues bearing that antigen, producing organ-specific disease (RBCs in hemolytic anemia, thyroid in Graves'). In Type III, antibodies bind soluble antigens in circulation, forming immune complexes that deposit wherever blood flows under high pressure — particularly renal glomeruli, synovial joints, and vessel walls — producing widespread, multi-organ inflammation unrelated to the antigen's original location.

This anatomical distinction has diagnostic and therapeutic implications. Type II diseases are organ-specific because the antibody is specific and its target is confined to one tissue. Type III diseases are systemic because complexes deposit non-specifically. Recognizing this difference guides clinical thinking: a patient with selective thyroid dysfunction and circulating anti-TSH receptor antibodies points to Type II; a patient with glomerulonephritis, arthritis, and skin lesions after drug exposure points to Type III.