Agglutination Reactions

Agglutination is defined as the antigen-antibody reaction in which antibodies cross-link particulate antigens resulting in the visible clumping of particles. Antibodies that show such reactions are called agglutinins.

Agglutination reactions work on the principle of cross-linking of the polyvalent antigens. Following are the advantages of agglutination reactions: 

1. easy to perform 
2. require no expensive equipment, and 
3. detects antibody concentrations as low as nanograms per milliliter. 

Types of agglutination reactions: 

1. Hemagglutination:
   type of agglutination reaction is routinely performed to type red blood cells (RBCs), wherein RBCs are mixed with antisera to the A or B blood-group antigens on a slide. The presence of antigen on the cell surface is proved by forming a visible clump on the slide. This RBC typing forms the basis for matching blood types for transfusions.
   
2. Bacterial Agglutination:

This type of agglutination reaction is performed to diagnose infection and provide a way to type bacteria. Any bacterial infection elicits the production of serum antibodies within the host. 

These serum antibodies are specific for surface antigens on the bacterial cells, which bacterial agglutination reactions can detect.

Bacteria is added to the previously serial diluted array of tubes containing serum from a patient thought to be infected with a given bacterium. The last tube, which shows visible agglutination, reflects the serum antibody titre of the patient. Thus, the agglutinin titre is the reciprocal of the greatest serum dilution that elicits a positive agglutination reaction.

3. Active agglutination 

In this type of agglutination, epitopes of interest are naturally found on a test particle, such as antigens found on RBCs, bacterial and fungal cells.

Examples –

1. Blood grouping and cross-matching
2. Widal test for diagnosis of typhoid fever 
3. Brucella agglutination test for Brucellosis 
4. Weil Felix test for Rickettsiosis
5. Passive agglutination: 

Passive agglutination is useful when the epitope of interest does not occur naturally on the cells or particles to be agglutinated. The epitopes or soluble antigens are chemically fixed to carrier particles such as – latex, polystyrene, bentonite.

 Passive agglutination is also useful when pathogen culture is not feasible, e.g., viral diseases. 

Synthetic beads offer better consistency, uniformity, and stability. In addition, those agglutination reactions which employ synthetic beads are rapidly read within 3 to 5 minutes of mixing the beads with the test sample.  

Article by- SAMPRATI PAREKH (MSIWM049).

References:

1. Cellular and Molecular Immunology by Abul K. Abbas – 7^thEdition 
2. Kuby Immunology – 5^thEdition 

Atopy

Synonym: Localized Anaphylaxis 

Introduction:

Atopy is defined as the tendency of an individual to produce IgE antibodies in response to various environmental antigens and thus develop strong immediate hypersensitivity (allergic) responses. Individuals with allergies to environmental antigens (e.g., pollen, house dust) are atopic. 

Localized anaphylaxis involves reactions limited to a specific target tissue or organ and often involves epithelial surfaces at the site of allergen entry. Atopy is thus defined as the tendency to manifest localized anaphylactic reactions, and this tendency is inherited. 

Atopic allergies include a wide range of IgE-mediated disorders, including allergic rhinitis (hay fever), asthma, atopic dermatitis (eczema), and food allergies. 

Allergic Rhinitis:

This is commonly known as “hay fever” and results from the reaction of airborne allergens with the sensitized mast cells in the conjunctivae and nasal mucosa, which induces the release of pharmacologically active mediators from mast cells. The mediators thus cause localized vasodilation and increased capillary permeability. The symptoms of allergic rhinitis usually include watery exudation of the conjunctivae, upper respiratory tract and nasal mucosa, and sneezing and coughing.

Asthma:

Asthma, a common manifestation of localized anaphylaxis, is triggered by degranulation of mast cells with the release of mediators, but instead of occurring in the nasal mucosa, the reaction develops in the lower respiratory tract. This results in the contraction of the bronchial smooth muscles and thus eventually leads to broncho-constriction. 

Food Allergies:

A variety of foods can induce localized anaphylaxis in allergic individuals. In addition, localized smooth-muscle contraction and vasodilation can be induced by allergen cross-linking of IgE on mast cells along the upper or lower gastrointestinal tract resulting in symptoms such as vomiting or diarrhea. 

Atopic Dermatitis:

Atopic dermatitis (allergic eczema) is an inflammatory disease of the skin frequently associated with a family history of atopy. This disease is observed most commonly among young children, often developing during infancy. Serum IgE levels are often elevated, and the allergic individual develops erythematous skin eruptions filled with pus. 

Article by- SAMPRATI PAREKH  (MSIWM049)

References:

1. Cellular and Molecular Immunology by Abul K. Abbas – 7^thEdition 
2. Kuby Immunology – 5^thEdition.