24.7
Allergic Reactions
Overview
We speak of an allergy when the immune system triggers a response against a benign foreign structure, like food, pollen or pet dander. These…
An allergy is a hypersensitive immune response to a normally harmless substance. The first process necessary for establishing an allergy is sensitization.
For example, individuals who are allergic to grass pollen have undergone sensitization. Their immune cells lining the respiratory passage take up and degrade the allergens into fragments.
These immune cells are called antigen presenting cells, or APCs because they display the degraded allergen fragments on their surface.
The APCs activate another encountered cell, type two T helper cells, abbreviated as Th2.
The activated Th2 then release chemical signals that cause another type of immune cell, B cells, to develop into antibody producing plasma cells which make immunoglobulin E, or IgE, a class of antibodies.
In this case, exposure to grass pollen will produce an IgE specific to this pollen, causing an allergic reaction distinct to grass pollen and no other type.
Once created, IgE binds to the surface of specialized cells called mast cells, found in tissues like the mucous membranes and skin.
Following sensitization, an allergic reaction occurs each time an individual encounters the allergen to which they are allergic after the allergen binds to its specific IgE on mast cells.
For instance, when the grass pollen allergen binds to its IgE, it causes the release of allergy symptom causing chemicals, such as leukotrienes and histamines that lead to sneezing, runny noses, and nasal passage inflammation.
Allergen exposure also causes mast cells and Th2 to release chemical signals that recruit and activate other inflammation inducing immune cells such as eosinophils and basophils to further amplify symptoms.
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Q1: What is sensitization and why is it the first step in developing an allergy?
Sensitization is the initial process that establishes an allergy. When antigen-presenting cells encounter an allergen, they degrade it into fragments and display these on their surface. These cells activate Type 2 helper T cells, which release signals causing B cells to produce immunoglobulin E (IgE) antibodies. IgE then binds to mast cells, priming the immune system for future allergic reactions to that specific allergen.
Q2: How do mast cells trigger allergic symptoms when exposed to an allergen?
When an allergen binds to IgE on mast cell surfaces, it stimulates mast cells to release inflammatory chemicals like histamines and leukotrienes. These mediators cause typical allergic symptoms such as sneezing, runny nose, and nasal inflammation. Additionally, mast cells and helper T cells release signals recruiting other immune cells like eosinophils and basophils, which amplify the inflammatory response.
Q3: What role do antigen-presenting cells play in the allergic response?
Antigen-presenting cells (APCs), including dendritic cells and macrophages, initiate sensitization by taking up allergens and degrading them into fragments. APCs display these fragments on their surface and activate Type 2 helper T cells, triggering the humoral immune response that produces IgE antibodies specific to the allergen.
Q4: Why is immunoglobulin E specific to individual allergens?
During sensitization, B cells produce IgE antibodies in response to specific allergen fragments presented by antigen-presenting cells. Each IgE molecule is tailored to recognize only its corresponding allergen. This specificity means grass pollen-specific IgE will trigger reactions only to grass pollen, not other allergens, ensuring targeted allergic responses.
Q5: What is anaphylactic shock and how does it differ from typical allergic reactions?
Anaphylactic shock is a rapid, systemic allergic reaction that can be life-threatening. Within minutes of allergen exposure, mast cells release large quantities of mediators into the bloodstream, causing airway restriction, blood vessel dilation, increased mucus production, and changes in heart rate. Unlike localized reactions, anaphylaxis affects multiple body systems and requires immediate epinephrine treatment to reverse these physiological changes.
Q6: What is cross-reactivity in allergic responses?
Cross-reactivity occurs when IgE antibodies produced against one allergen trigger an allergic reaction to a different allergen. For example, ragweed pollen-specific IgE might also react to structurally similar allergens. This phenomenon explains why individuals allergic to one substance may experience reactions to seemingly unrelated allergens.
Q7: How does epinephrine treat anaphylactic shock?
Epinephrine is the only effective medication for anaphylaxis. It counteracts the complex physiological changes by constricting blood vessels, increasing heart rate, stabilizing heart contractility, and increasing airflow through the airways. Rapid administration via EpiPen is critical, as quick response within minutes of allergen exposure significantly improves survival outcomes.