Type I Hypersensitivity:

Immune responses normally operate as defense mechanisms that eliminate potentially harmful antigens from the host. However, in certain circumstances, the antigen persists or the immune response becomes dysregulated, resulting in host tissue damage. This pathological state is known as hypersensitivity, defined as an exaggerated immune response to a typically harmless antigen that results in tissue injury, disease, or even death.

In the $1960\text{s}$, British immunologists Philip Gell and Robin Coombs established a classification system categorizing hypersensitivity reactions into four major types based on their underlying immunologic mechanisms:

• Type I Hypersensitivity: Also known as anaphylactic hypersensitivity.

• Type II Hypersensitivity: Also known as antibody-mediated cytotoxic hypersensitivity.

• Type III Hypersensitivity: Also known as complex-mediated hypersensitivity.

• Type IV Hypersensitivity: Also known as cell-mediated hypersensitivity.

Hypersensitivity types I, II, and III are mediated by antibodies. Because the clinical signs and symptoms of these antibody-mediated reactions manifest rapidly—within a few minutes to a few hours following exposure to the inducing antigen—they are designated as immediate hypersensitivity reactions.

In contrast, Type IV hypersensitivity is mediated by sensitized T cells rather than antibodies. The clinical manifestations of type IV hypersensitivity do not appear until $24\text{ to }48\,\text{hours}$ following contact with the antigen; consequently, this category is termed delayed hypersensitivity.

Type I Hypersensitivity and Atopy

Type I hypersensitivity reactions are clinically recognized as allergies. Manifestations include:

• Hay fever (allergic rhinitis)

• Allergic asthma

• Hives (urticaria)

• Systemic anaphylaxis

The specific antigens that induce type I hypersensitivity reactions are termed allergens. Common examples include peanuts, eggs, and pollen. The cardinal feature of type I hypersensitivity is the rapid onset of clinical symptoms, which typically develop within minutes of allergen exposure.

Patients who develop allergic hypersensitivity reactions characteristically generate large quantities of immunoglobulin E ($IgE$) antibodies in response to minute concentrations of allergen. Susceptibility to elevated $IgE$ levels is governed by an interplay between genetic predisposition and environmental factors.

Atopy (derived from the Greek term atopos, meaning "out of place") refers to an inherited tendency to develop classic allergic responses to naturally occurring inhaled or ingested environmental allergens.

Discovery and Biology of Immunoglobulin E (IgE)

The Prausnitz-Küstner Experiment

The initial evidence that a serum factor mediates type I hypersensitivity was demonstrated in $1921$ by Carl Wilhelm Prausnitz and Heinz Küstner. Küstner possessed an allergy to fish. In their experiment, serum from Küstner was injected intradermally into Prausnitz (who was not allergic to fish). Subsequent exposure of the injected site to fish antigen elicited localized redness and swelling.

This experimental reaction is termed passive cutaneous anaphylaxis, defined as the process wherein serum from an allergic individual is transferred into a non-allergic recipient, who is subsequently challenged with the specific allergen at the site of transfer. In $1967$, the serum factor responsible for passive cutaneous anaphylaxis was definitively identified as immunoglobulin E (IgE).

Structural and Functional Characteristics of IgE

• Serum Abundance: $IgE$ is the least abundant antibody class in human serum, normally accounting for less than $1\%$ of total circulating immunoglobulins. This low serum concentration correlates with its exclusion from classical humoral immune effector functions, such as complement fixation and classical opsonization.

• Receptor Binding Affinity: $IgE$ possesses the unique biological capability to bind with high affinity to specific receptors ($Fc\varepsilon RI$) present on the plasma membranes of mast cells and basophils.

• Physiological Roles: Beyond mediating type I hypersensitivity allergic reactions, $IgE$ serves a primary host-defense function against parasitic infections. Patients suffering from parasitic infestations or severe allergies exhibit markedly elevated serum concentrations of $IgE$.

• Membrane Half-Life Extension: In free serum, unbound $IgE$ exhibits a half-life of only $2\text{ to }3\,\text{days}$. However, once bound to high-affinity receptors on effector cell membranes, the half-life of $IgE$ is extended to at least $10\,\text{days}$, allowing it to serve as a stable, persistent antigen receptor on cell surfaces.

Immunologic Mechanism of Type I Hypersensitivity

Type I hypersensitivity involves four key immunologic components: IgE antibodies, mast cells, basophils, and eosinophils. These cellular and humoral elements cooperate to trigger an immediate reaction, typically occurring within $30\text{ to }60\,\text{minutes}$ following allergen challenge.

The pathophysiology of type I hypersensitivity is divided into two primary phases: the sensitization phase and the activation phase. In many individuals, this is followed by a late-phase reaction.

The Sensitization Phase

Sensitization initiates upon the first exposure of a genetically susceptible individual to an environmental allergen:

1. Antigen Uptake and Presentation: Specialized antigen-presenting cells (APCs), specifically Langerhans cells and tissue dendritic cells, internalize and process environmental allergens. These cells process the allergen into peptides and present them on the cell surface bound to major histocompatibility complex class II ($MHC\text{ class II}$) molecules. The APCs migrate to local lymphoid tissues (such as regional lymph nodes) to interact with naive T helper cells.

2. Th2 Cell Differentiation: In non-atopic individuals, an appropriate balance between type 1 helper T cells ($Th1$) and type 2 helper T cells ($Th2$) maintains protective immunity without host injury. In allergic individuals, the response is skewed toward a predominant $Th2$ phenotype.

3. Cytokine Signaling: Activated $Th2$ cells secrete key cytokines, notably interleukin-4 ($IL-4$) and interleukin-13 ($IL-13$).

4. B-Cell Class Switching: $IL-4$ and $IL-13$ stimulate local B lymphocytes and plasma cells—predominantly located in the lymphoid tissue of the respiratory and gastrointestinal tracts and regional lymph nodes—to undergo immunoglobulin heavy chain class switching. Specifically, these cytokines induce transcription of the gene encoding the epsilon ($\varepsilon$) heavy chain, driving synthesis of allergen-specific $IgE$.

5. Receptor Loading: The synthesized $IgE$ molecules enter the circulation and bind via their fragment crystallizable ($Fc$) regions to high-affinity $Fc\varepsilon RI$ receptors on tissue mast cells and circulating basophils. A single mast cell or basophil expresses as many as $200{,}000$ of these surface $Fc\varepsilon RI$ receptors. The bound $IgE$ molecules function thereafter as specific cell-surface antigen receptors.

Effector Cells: Mast Cells and Basophils

Mast Cells

Mast cells are the primary effector cells of immediate hypersensitivity:

• Tissue Distribution: Ubiquitous throughout vascularized tissues, concentrated around small blood vessels, lymphatic vessels, peripheral nerves, and glandular tissue.

• Morphology: Packed with abundant cytoplasmic granules containing preformed inflammatory mediators.

• Longevity: Long-lived cells that persist in peripheral tissues for months.

Basophils

Basophils share functional similarities with mast cells but represent a distinct lineage:

• Distribution: Circulate in peripheral blood, representing less than $1\%$ of total circulating white blood cells ($WBCs$).

• Morphology: Possess fewer, but physically larger, cytoplasmic granules compared to mast cells. The biochemical concentrations and compositions of inflammatory mediators within basophil granules differ from those in mast cells.

• Kinetics: Migrate into tissues in response to chemotactic stimuli during inflammatory events, where they persist for several days.

The Activation Phase and Primary Mediators

Upon subsequent exposure to the allergen, sensitized mast cells and basophils undergo activation and degranulation:

1. Receptor Cross-Linking: Multivalent or bivalent allergens bind simultaneously to adjacent membrane-bound $IgE$ molecules on the surface of sensitized effector cells. This cross-linking aggregates the underlying $Fc\varepsilon RI$ surface receptors.

2. Intracellular Signaling Cascade: Receptor aggregation initiates complex intracellular biochemical events, including protein phosphorylation cascades, an influx of extracellular calcium ($Ca^{2+}$), and activation of gene transcription for cytokine synthesis.

3. Degranulation: The abrupt increase in intracellular free calcium triggers the fusion of preformed cytoplasmic granules with the plasma membrane, resulting in the rapid exocytosis of chemical mediators into the extracellular microenvironment.

Primary (Preformed) Mediators

Primary mediators are synthesized prior to cell activation and stored within secretory granules for instantaneous release:

• Histamine: The most abundant preformed mediator, accounting for approximately $10\%$ of the total granule dry weight in mast cells. Histamine mediates smooth muscle contraction, arteriolar vasodilation, and increased postcapillary venular permeability.

• Heparin: An acidic proteoglycan that acts alongside histamine to promote smooth muscle contraction, vasodilation, and increased vascular permeability.

• Eosinophil Chemotactic Factor of Anaphylaxis (ECF-A): Selectively attracts circulating eosinophils to the site of allergic activation.

• Neutrophil Chemotactic Factor of Anaphylaxis (NCF-A): Directs the recruitment and migration of circulating neutrophils to the inflammatory focus.

• Proteases (e.g., Tryptase, Chymase): Cleave complement component $C3$ into active $C3b$, promote local mucus secretion, and activate surrounding pro-inflammatory cytokines.

Early-Phase Clinical Manifestations

The biological actions of primary mediators generate early-phase symptoms within $30\text{ to }60\,\text{minutes}$ of exposure. Symptoms depend on the target tissue and the local receptor profiles:

• Cutaneous: Dermal exposure leads to local edema and erythema, forming a characteristic wheal-and-flare reaction.

• Respiratory: Bronchial smooth muscle constriction accompanied by mucosal edema and hypersecretion results in acute airflow obstruction and wheezing.

• Systemic Vasculature: Widespread endothelial permeability and massive vasodilation induce a rapid drop in total peripheral resistance, leading to systemic hypotension or anaphylactic shock.

Comprehensive Mediator Actions

The Late-Phase Reaction and Tissue Remodeling

In addition to releasing preformed granules, activation of mast cells and basophils triggers the enzymatic breakdown of membrane phospholipids, initiating de novo synthesis of secondary mediators:

• Eicosanoids: Phospholipase $A_2$ activation liberates arachidonic acid, which is metabolized via the cyclooxygenase pathway into prostaglandin $PGD_2$ and via the 5-lipoxygenase pathway into leukotrienes ($LTB_4$, $LTC_4$, $LTD_4$, and $LTE_4$).

• Platelet-Activating Factor (PAF): Derived from lyso-PAF, triggering platelet aggregation and potent vasoactive changes.

• Cytokines: De novo synthesis of interleukins and growth factors ($IL-1$, $IL-3$, $IL-4$, $IL-5$, $IL-6$, $IL-9$, $IL-13$, $IL-14$, $IL-16$, $TNF\text{-}\alpha$, and $GM\text{-}CSF$).

Secondary mediators are significantly more potent on a molar basis than primary mediators. They drive the late-phase reaction, which emerges in susceptible individuals $6\text{ to }8\,\text{hours}$ following initial antigen contact.

Cellular Infiltration and the Role of Eosinophils

During the late-phase reaction, circulating leukocytes exit the vasculature and infiltrate allergen-exposed tissues. Infiltrating populations include eosinophils, neutrophils, $Th2$ cells, basophils, macrophages, and additional mast cells.

Eosinophils are central effectors of chronic allergic inflammation:

• Baseline Physiology: Eosinophils normally comprise only $1\%\text{ to }3\%$ of circulating peripheral $WBCs$.

• Cytokine-Driven Eosinophilia: Interleukin-5 ($IL-5$) and associated cytokines secreted by activated $Th2$ cells act on hematopoietic progenitors in the bone marrow, stimulating eosinophil differentiation and output into the peripheral blood (producing eosinophilia).

• Receptor Upregulation and Degranulation: Activated eosinophils upregulate cell-surface $Fc\varepsilon RI$ expression and release toxic granule constituents, reactive oxygen species, and secondary cytokines directly into inflamed tissues, causing persistent tissue damage.

Tissue Remodeling

Persistent, unresolved late-phase inflammation leads to permanent architectural alterations termed tissue remodeling (commonly seen in chronic allergic asthma). Pathological structural modifications include:

• Hypertrophy and hyperplasia of bronchial smooth muscle layers

• Fibrotic changes and alterations within connective tissue matrices

• Neovascularization of blood vessels and changes in lymphatic architecture

• Hyperplasia of submucosal mucus glands and goblet cells, resulting in excessive mucus production

• Structural alterations and hypersensitivity of local autonomic nerves

Genetic and Environmental Influences on Atopy

The development of clinical allergy and sustained $IgE$ hyperproduction is multifactorial, governed by complex gene-environment interactions. Several hundred candidate susceptibility genes have been identified across two primary functional pathways:

1. Epithelial Barrier Integrity: A cluster of susceptibility genes encodes structural proteins of the epithelial lining across entry portals (skin, gastrointestinal tract, and respiratory tract). Polymorphic variants in these genes impair barrier integrity, allowing unhindered penetration of potential environmental allergens and microbial components into deeper mucosal tissues.

2. Innate Immune Sensing: A second group of genetic variants affects pattern recognition receptors and downstream innate signaling pathways, determining how foreign antigens are recognized and presented to helper T cells following barrier breach.