Situation: A 26-year-old woman with systemic lupus erythematosus (SLE) comes to the medical outpatient clinic for follow-up. She takes hydroxychloroquine daily and prednisone 5 mg daily. Her usual blood pressure is 110/70 mmHg. Which type of hypersensitivity reaction causes the tissue damage in SLE?

In SLE, antigen–antibody (immune) complexes deposit in vessels, kidneys, joints, and skin and trigger inflammation, which is a type III reaction. Serum sickness and post-streptococcal glomerulonephritis are other type III examples. Type II reactions, such as myasthenia gravis, involve antibodies directed at cell surface receptors.

Core mechanism
Systemic lupus erythematosus is the classic example of a type III hypersensitivity reaction. The tissue injury begins when autoantibodies bind circulating self-antigens—especially nuclear material such as double-stranded DNA and nucleosomes—to form soluble antigen–antibody immune complexes. Because these complexes are small and soluble, they are not cleared efficiently. They deposit in vascular walls, renal glomeruli, joints, and skin, where they activate complement and attract neutrophils. The resulting inflammation and enzymatic damage produce the multisystem findings of SLE ^[1].

Immune complex deposition, not direct antibody attack on a fixed cell surface, is the central driver of organ damage in SLE. This is why the disease can involve so many different tissues at the same time.

Hypersensitivity type	Key effector	Clinical example	SLE relevance
Type I	IgE, mast cells, histamine	Anaphylaxis, allergic rhinitis	Not the mechanism of SLE tissue damage
Type II	IgG or IgM against cell surface or matrix antigens	Myasthenia gravis, autoimmune hemolytic anemia	Autoantibodies exist, but organ damage is not primarily from direct cytotoxicity
Type III	Soluble immune complexes, complement, neutrophils	SLE, serum sickness, post-streptococcal glomerulonephritis	Primary mechanism of tissue injury
Type IV	Sensitized T cells, macrophages	Contact dermatitis, tuberculin reaction	Not the principal pathway in SLE

Watch out! SLE patients do produce autoantibodies such as anti-dsDNA and antinuclear antibodies, which can tempt a choice of type II. However, the presence of autoantibodies alone does not define type II. In type II, the antibody binds directly to a fixed cell surface or tissue antigen and causes damage at that site. In SLE, the antibodies bind soluble nuclear antigens, and the resulting immune complexes travel and deposit elsewhere—that is type III ^[1].

The distinction between type II and type III depends on whether the antibody attacks a fixed tissue antigen directly or forms a circulating complex that deposits secondarily. Myasthenia gravis illustrates type II because antibodies target acetylcholine receptors on the motor end plate. SLE illustrates type III because the injury follows complex deposition in vessels and glomeruli.

Key point! When a licensure question asks for the hypersensitivity mechanism of SLE, the expected answer is type III, immune complex deposition. Other type III examples—serum sickness and post-streptococcal glomerulonephritis—reinforce the same pattern of circulating complex deposition followed by complement-mediated inflammation ^[1].