Hypersensitivity, Autoimmunity, and Immunodeficiency

1. Hypersensitivity: Four Types

Hypersensitivity is an exaggerated or misdirected immune response that damages tissue. Type I (immediate) hypersensitivity is IgE-mediated and occurs within minutes — mast cells release histamine and other mediators, producing allergy, hives, and in severe cases anaphylaxis. Type II (cytotoxic) hypersensitivity involves IgG or IgM antibodies binding to cell-surface antigens and destroying those cells. Type III (immune-complex) hypersensitivity involves antigen–antibody complexes depositing in tissues and triggering damaging inflammation. Type IV (delayed-type) hypersensitivity is T-cell–mediated and takes 24–72 hours to develop — it does not involve antibodies.

2. Autoimmunity and Autoimmune Disease

Autoimmunity is an immune response directed against the body's own antigens. Autoimmune disease occurs when that response causes tissue damage and clinical illness. Immune dysregulation — a failure of the checks that normally keep the immune system from attacking self — underlies autoimmunity. Autoimmune diseases are generally described by their mechanism and target tissue (for example, conditions affecting joints, the thyroid, or the nervous system), and their diagnosis and management are the responsibility of qualified clinicians.

3. Immunodeficiency

Immunodeficiency is a state of reduced immune function that leaves a person more susceptible to infection. Primary immunodeficiency is present from birth, typically due to inherited defects in immune components. Acquired (secondary) immunodeficiency develops later in life from causes such as certain infections, malnutrition, or some medical treatments. Immune dysregulation links these states to the other side of the spectrum: an immune system that is underactive (immunodeficiency) or overactive or misdirected (hypersensitivity and autoimmunity) both reflect a loss of normal balance.

How it works

1. In Type I hypersensitivity, a first exposure produces IgE that binds to mast cells; re-exposure to the allergen triggers rapid mediator release and symptoms within minutes.
2. In Type II, IgG or IgM antibodies bind cell-surface antigens and mark the cells for destruction.
3. In Type III, antigen–antibody complexes form in circulation and deposit in tissues, causing inflammation.
4. In Type IV, sensitized T cells respond to antigen over 24–72 hours, causing delayed tissue damage.
5. In autoimmunity, a breakdown in self-tolerance lets the immune system attack the body's own tissues.
6. In immunodeficiency, missing or impaired immune components — from birth or acquired later — leave the person vulnerable to infection.

Common confusions

Memory aids

For the four hypersensitivity types, remember "A C I D" by mechanism — Allergy (Type I, IgE), Cytotoxic (Type II, antibody on cells), Immune complexes (Type III), Delayed T cells (Type IV). The order spells "ACID," and they follow the sequence antibody (I–III) then cell (IV).

Quick review

Topic Recap

The immune system can harm as well as protect. Hypersensitivity is an excessive or misdirected response in four forms — immediate IgE-mediated allergy (Type I), antibody-mediated cell damage (Type II), immune-complex deposition (Type III), and delayed T-cell responses (Type IV). Autoimmunity turns the response against self, and immunodeficiency reduces protective capacity, either from birth or acquired later. Recognizing these patterns, communicating accurately, and respecting referral boundaries are the core clinical lessons.

Knowledge Check

1. Which antibody mediates Type I hypersensitivity?
2. How does Type II differ from Type III hypersensitivity?
3. What is the difference between autoimmunity and autoimmune disease?
4. Distinguish primary from acquired immunodeficiency.
5. Why do referral boundaries matter when these conditions are suspected?

Answers and Rationales

1. IgE — Type I is an immediate, IgE-mediated reaction involving mast cells.
2. Type II uses antibodies to attack cells directly; Type III forms antigen–antibody complexes that deposit in tissues and cause inflammation.
3. Autoimmunity is an immune response against self-antigens; autoimmune disease is that response causing tissue damage and clinical illness.
4. Primary immunodeficiency is present from birth (often inherited); acquired immunodeficiency develops later from external causes such as infection, malnutrition, or certain treatments.
5. Because diagnosis and management require qualified clinical judgment — recognizing a pattern and escalating it within one's role, following local policy, protects patients and respects professional limits.