Fundamental Differences in Hypersensitivity Reactions
📌 There are four main types of hypersensitivity reactions, with a fifth type sometimes mentioned but mechanistically grouped with the second.
🔗 Types I, II, and III are antibody-mediated (humoral immunity), while Type IV is T-cell mediated (cellular immunity) and does not require antibodies.
🚫 Types I and II reactions result in antigen-antibody complexes that do not circulate in the bloodstream because either the antigen (Type II) or the antibody (Type I) is fixed.
🔄 Type III reactions involve free-floating antigen-antibody complexes (both components are free), allowing them to circulate and cause damage in various tissues.

Type I Hypersensitivity Mechanism (Immediate Allergy)
🤧 In Type I reactions, the antigen is typically free and foreign (e.g., inhaled pollen), triggering a humoral response leading to the production of Immunoglobulin E (IgE).
🛡️ IgE antibodies fix themselves to mast cells via their Fc fragment, sensitizing them.
💥 Upon re-exposure to the same antigen, the antigen binds to the fixed IgE, causing mast cell degranulation and the release of inflammatory mediators like histamine.
🧪 Mast cell degranulation releases primary mediators (e.g., histamine, serotonin) which are pre-formed, and stimulates the release of secondary mediators (e.g., cytokines like $\text{IL-}1, \text{IL-}3, \text{IL-}4, \text{IL-}5, \text{IL-}6$), requiring gene activation.

Mediator Effects and Clinical Manifestations
💨 Histamine causes vasodilation (increasing blood flow), increased vascular permeability (leading to edema/swelling), and a spasmogenic effect on smooth muscles (narrowing airways).
🩸 Secondary mediators, especially $\text{IL-}3$ and $\text{IL-}5$, strongly attract eosinophils and neutrophils to the site of inflammation.
🔬 The overall effect of these mediators leads to symptoms like dyspnea (shortness of breath) due to airway narrowing and edema, characteristic of conditions like allergic rhinitis, conjunctivitis, hay fever, and anaphylactic shock.

Diagnosis of Type I Hypersensitivity
🧪 Prick Test (Intradermal Test): Involves introducing a small amount of allergen directly into or onto the skin to observe a localized reaction (redness/wheal) if the person is sensitized.
🩸 RAST (Radioallergosorbent Test): A blood test used to detect and quantify IgE antibodies specific to certain allergens circulating in the patient's serum.

Key Points & Insights
➡️ Type I allergy is characterized by free antigen and IgE fixed on mast cells; re-exposure causes immediate degranulation.
➡️ The goal of mast cell mediators (like histamine) is to achieve vasodilation, increased permeability, smooth muscle contraction (spasmogenic effect), and increased mucus secretion.
➡️ $\text{IL-}4$ is crucial for B-lymphocyte proliferation, while $\text{IL-}5$ catalyzes the transformation of B-cells into plasma cells that synthesize IgE.
➡️ Anaphylactic shock is a life-threatening systemic reaction caused by widespread mast cell degranulation leading to severe circulatory collapse (intense vasodilation).

📸 Video summarized with SummaryTube.com on Oct 08, 2025, 19:39 UTC