Pathology
Immunopathology and AIDS
Immunopathology for MBBS and NEET-PG: four hypersensitivity reactions, autoimmunity and lupus, HLA and transplant rejection, and HIV and AIDS pathology, mapped to NMC codes PA9.1 to PA9.7.
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Immunopathology and AIDS
Immunopathology for MBBS and NEET-PG: four hypersensitivity reactions, autoimmunity and lupus, HLA and transplant rejection, and HIV and AIDS pathology, mapped to NMC codes PA9.1 to PA9.7.
This chapter covers immune-mediated tissue injury and immunodeficiency. It sets out the four types of hypersensitivity, the principles of autoimmunity and systemic lupus erythematosus, the HLA system and transplant rejection, and the pathogenesis, opportunistic infections and tumours of HIV and AIDS.
High-yield: Immunopathology and AIDS
- Type I hypersensitivity is IgE-mediated and immediate, causing anaphylaxis, atopy, asthma and urticaria.
- Type II hypersensitivity is antibody-mediated cytotoxicity, as in autoimmune haemolytic anaemia and Goodpasture syndrome.
- Type III hypersensitivity is immune complex-mediated, as in systemic lupus erythematosus, serum sickness and post-streptococcal glomerulonephritis.
- Type IV hypersensitivity is delayed and T cell-mediated, as in the tuberculin reaction, contact dermatitis and graft rejection.
- Anaphylaxis results from mast cell degranulation releasing histamine and is treated with adrenaline.
- Autoimmunity arises when tolerance to self-antigens fails, and shows female predominance in most conditions.
- Antinuclear antibody is the screening test for systemic lupus erythematosus; anti-double-stranded DNA and anti-Smith are specific.
- The lupus band test shows immunoglobulin and complement deposition at the dermo-epidermal junction.
- HLA genes on chromosome 6 encode the major histocompatibility complex and determine transplant compatibility.
- Hyperacute graft rejection occurs within minutes due to preformed antibodies; acute rejection occurs within days to weeks; chronic rejection develops over months to years.
- HIV is a retrovirus that infects CD4-positive T cells using the CD4 receptor and CCR5 or CXCR4 co-receptors.
- AIDS is defined by a CD4 count below 200 cells per microlitre or by an AIDS-defining illness.
- Pneumocystis jirovecii pneumonia, oesophageal candidiasis and cerebral toxoplasmosis are common opportunistic infections in AIDS.
- Kaposi sarcoma (linked to HHV-8) and non-Hodgkin lymphoma are the characteristic malignancies of AIDS.
- HIV is diagnosed by ELISA for screening and confirmed by Western blot, with viral load and CD4 count used to monitor disease.
The four hypersensitivity reactions
- **Type I:** IgE-mediated immediate; anaphylaxis, atopy, asthma.
- **Type II:** Antibody-mediated cytotoxicity; autoimmune haemolysis, Goodpasture syndrome.
- **Type III:** Immune complex; lupus, serum sickness, post-streptococcal glomerulonephritis.
- **Type IV:** Delayed, T cell-mediated; tuberculin test, contact dermatitis, graft rejection.
NMC competencies in this chapter
- **PA9.1:** Principles and mechanisms of immunity
- **PA9.2:** Mechanisms of hypersensitivity reactions
- **PA9.3:** HLA system, transplant immunology and mechanisms of graft rejection
- **PA9.4:** Definition of autoimmunity and enumeration of autoimmune disorders
- **PA9.5:** Pathogenesis of systemic lupus erythematosus
- **PA9.6:** Pathogenesis and pathology of HIV and AIDS
- **PA9.7:** Pathogenesis of other common autoimmune diseases
Frequently Asked Questions
How are the hypersensitivity reactions classified?
Type I is IgE-mediated and immediate, type II is antibody-mediated cytotoxic, type III is immune complex-mediated, and type IV is delayed and T cell-mediated.
What defines AIDS?
AIDS is defined by a CD4 count below 200 cells per microlitre or the presence of an AIDS-defining opportunistic infection or tumour in an HIV-positive person.
Which antibodies are specific for lupus?
Anti-double-stranded DNA and anti-Smith antibodies are specific for systemic lupus erythematosus, while the antinuclear antibody is a sensitive screening test.
What cell does HIV infect?
HIV infects CD4-positive T helper cells using the CD4 receptor together with the CCR5 or CXCR4 co-receptor, leading to progressive immune failure.
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