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.

  1. 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.  




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


Synonym: Localized Anaphylaxis 


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, 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. 



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

Antigen presenting cells


The population of specialized cells: 

  1. to capture microbial and other antigens,
  2. display MHC complexes in association with these peptide fragments of protein antigens on its surface to lymphocytes, and
  3. provide signals that stimulate the proliferation and differentiation of the lymphocytes (co-stimulatory signal) are known as the antigen-presenting cells (APCs). 

APCs are conventionally referred to as those cells which display antigens on their surface to the T lymphocytes. A dendritic cell is the major type of APC which is involved in initiating the T cell responses. 

Macrophages and B cells also present antigens to the T lymphocytes but in different types of immune responses. The follicular dendritic cell, a specialized cell type, displays antigens to B lymphocytes during particular phases of humoral immune responses. APCs thus link responses of the innate immune system to responses of the adaptive immune system, and therefore they may be considered components of both systems. 

Types of APCs: 

  1. Dendritic Cells

Dendritic cells form one of the most important APCs for activating naive T cells. These cells constitutively express a high level of class II MHC molecules and deliver a co-stimulatory activity and thus play major roles in innate responses to infections and link innate and adaptive immune responses.

  1. Antigen-Presenting Cells for Effector T -Lymphocytes

In addition to dendritic cells; macrophages and B lymphocytes perform important antigen-presenting functions in CD4+ helper T cell-mediated immune responses. 

  1. Macrophages present antigen to helper T lymphocytes at the sites of infection, which leads to helper T cell activation and production of molecules that further activate the macrophages. These macrophages must be activated by phagocytosis of particulate antigens before expressing class II MHC molecules or the co-stimulatory B7 membrane molecule.
  1. B cells present antigens to helper T cells in lymph nodes and spleen, a key step in the cooperation of helper T cells with B cells in humoral immune responses to protein antigens. These B cells constitutively express class II MHC molecules but must be activated before expressing the co-stimulatory B7 molecule. 

Note: Cytotoxic T lymphocytes (CTLs) are effector CD8+ T cells that can recognize antigens on any nucleated cell and become activated to kill the cell. Thus, all nucleated cells are potentially APCs for CTLs. 

  1. Follicular Dendritic Cells: 

Follicular dendritic cells (FDCs) are cells with membranous projections found intermingled in specialized collections of activated B cells, called germinal centers, in the lymphoid follicles of the lymph nodes, spleen, and mucosal lymphoid tissues. 



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


Synonym: Programmed cell death

Pronunciation: ah-poh-toh’-sis: Greek, (1) apo- away from and (2) ptosis, a falling or dropping 

Apoptosis’ has been derived from a Greek word that describes the falling of the leaves from a tree or petals from a flower. This term was coined to differentiate this form of programmed cell death from the accidental cell deaths caused by inflammation or injury (necrosis). 


Programmed cell death is an active process, first described in 1972 and usually proceeds by a distinct series of cellular changes known as apoptosis. 

Apoptosis is thus the genetically programmed death of cells that is both a natural development process and the body’s means of destroying abnormal or infected cells. 

Events of apoptosis:

During apoptosis, firstly, as a result of cleavage between nucleosomes, chromosomal DNA fragmentation occurs. 

Following chromatin condensation, the cell shrinks in and breaks up into membrane-enclosed fragments known as apoptotic bodies. 

These apoptotic cells and cell fragments are efficiently recognized and phagocytosed by macrophages and the neighbouring cells, and thus, eventually, these cells, which die by apoptosis, are rapidly removed from tissues. 


This removal of the apoptotic cells from the tissues is mediated by the expression of certain signals on the cell surface. These signals are generally known as the “eat me” and include phosphatidylserine, which is restricted normally towards the inner leaflet of the plasma membrane. However, during apoptosis, phosphatidylserine becomes expressed on the cell surface, where it is recognized by receptors expressed by phagocytic cells. 

Genes involved in apoptosis:

Programmed cell death was innovatively studied during the development of C. elegans, which eventually provided the critical initial insights that led to understanding the molecular mechanism of apoptosis. These pioneering studies conducted in the laboratory of Robert Horvitz helped initially identify three genes that played key roles in regulating and executing apoptosis. 

Mutagenesis of C. elegans in the year 1986 helped identify the genes involved in the developmental cell death (ced-3and ced-4). If either ced-3 or ced-4 was inactivated by mutation, the normally programmed cell deaths did not occur. 

A third gene known as the ced-9 functioned as a negative regulator of apoptosis. Whenever this gene ced-9 was inactivated by mutation, the cells failed to survive and instead underwent apoptosis, leading to death. Conversely, if this gene was expressed at an abnormally high level, the normally programmed cell deaths failed to occur.



  1. The Cell: A Molecular Approach by Geoffrey M. Cooper – 8thEdition 
  2. Cellular and Molecular Immunology by Abul K. Abbas – 7thEdition 



Aminoglycosides comprise a complex group of drugs derived from soil Actinomycetes in the genera Streptomyces and Micromonospora that impairs ribosome function and has antibiotic potential. 

Examples includes Streptomycin, gentamicin, tobramycin, and, amikacin.

Mode of Action:

This complex group of drugs inserts itself on sites on the 30S ribosomal subunit of the prokaryotes and causes the misreading of the mRNA, eventually leading to abnormal proteins. 

Figure: Site of inhibition on the prokaryotic ribosome by the drug aminoglycoside, which has a general effect of blocking the protein synthesis. The blockage action is indicated by X. 

Structure of the drug:

The aminoglycoside class of drugs comprises one or more amino sugars and an aminocyclitol ring, a 6-carbon cyclic ring. 

Figure: The structure of an aminoglycoside: Streptomycin.

 Coloured portions of the molecule are found in all members of this drug class. 

Subgroups and Uses of Aminoglycosides:

The aminoglycoside group of drugs possesses a relatively broad antimicrobial spectrum since they inhibit the synthesis of prokaryotic proteins by binding to one of the ribosomal subunits. Therefore, this group of drugs is mostly used to treat infections generally caused by aerobic gram-negative rods and gram-positive bacteria. 

Streptomycin is one of the oldest known drugs. However, it is gradually being replaced by newer forms of drugs that possess less mammalian toxicity. However, even today, Streptomycin is considered an effective antituberculosis agent and an antibiotic of choice for treating bubonic plague and tularaemia. 

Gentamicin is less toxic and is widely administered for infections caused by gram-negative rods such as Escherichia, Pseudomonas, Salmonella, and Shigella. 

Two relatively new aminoglycosides; amikacin, and tobramycin are also used for gram-negative infections, with tobramycin especially useful for treating Pseudomonas infections in cystic fibrosis patients.

Antimicrobial Resistance 

Since this complex group of drugs works by blocking the protein synthesis in prokaryotes, the microbes usually circumvent these drugs by altering the nature of the protein target. Bacteria can thus become resistant to aminoglycosides when point mutations in ribosomal proteins arise.

Drug toxicity:

Aminoglycosides can directly act on the brain and cause seizures. In addition, this group of drugs may damage nerves (very commonly, the eighth cranial nerve), leading to dizziness, deafness, or motor and sensory disturbances. 

Aminoglycosides such as gentamicin are nephrotoxic and are poorly cleared by damaged kidneys. 

Note: The intake of other drugs must be carefully scrutinized because incompatibilities can result in increased toxicity or failure of one or more of the drugs. For example, the combination of aminoglycosides and cephalosporins increases nephrotoxic effects. 


References: Talaros Foundations in Microbiology – 8thEdition