Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Immumogenetics

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Immunofluorescence microscopy can revel much detail

about the processes inside cells. In a light microscopic applica-

tion of the technique, sections of sample are exposed to the flu-

orescently labeled antibody. The large wavelength of visible

light, relative to other forms of illumination such as laser light,

does not allow details to be revealed at the molecular level.

Still, details of the trafficking of a protein from the site of its

manufacture to the surface of a cell, for example, is possible,

by the application of different antibodies. The antibodies can

be labeled with the same fluorescent compound but are applied

at different times. An example of the power of this type of

approach is the information that has been obtained concerning

the pathway that the yeastknown as Saccharomyces cervisiae

uses to shuttle proteins out of the cell.

Resolution of details to the molecular level has been

made possible during the 1990s with the advent of the tech-

nique of confocal laser microscopy. This technique employs a

laser to sequentially scan samples at selected depths through

the sample. These so-called optical sections can be obtained

using laser illumination at several different wavelengths

simultaneously. Thus, the presence of different antibodies that

are labeled with fluorescent compounds that fluoresce at the

different wavelengths can produce an image of the location of

two antigens in the same sample at the same time.

The use of immunofluorescent compounds in combina-

tion with confocal microscopy has allowed the fluorescent

probing of samples which do not need to be chemically pre-

served (or “fixed”) prior to examination. The thin sections of

sample that are examined in light microscopy often require

such chemical fixation. While the fixation regimens have been

designed to avoid change of the sample’s internal structure,

especially the chemistry and three-dimensional structure of

the site of the antigen to which the antibody will bind, the

avoidance of any form of chemical modification is preferred.

There are a multitude of fluorescent compounds avail-

able. Collectively these compounds are referred to as fluo-

rochromes. A well-known example in biological and

microbiological studies is the green fluorescent protein. This

molecule is ring-like in structure. It fluoresces green when

exposed to light in the ultraviolet or blue wavelengths. Other

compounds such as fluorescein, rhodamine, phycoerythrin,

and Texas Red, fluoresce at different wavelength and can pro-

duce different colors.

Immunofluorescence can be accomplished in a one-step

or two-step reaction. In the first option, the fluorescently

labeled antibody directly binds to the target antigen molecule.

In the second option the target antigen molecule binds a so-

called secondary antibody. Then, other antigenic sites in the

sample that might also bind the fluorescent antibody are

“blocked” by the addition of a molecule that more globally

binds to antigenic sites. The secondary antibody then can itself

be the target to which the fluorescently labeled antibody binds.

The use of antibodies to antigen that are critical to dis-

ease processes in microorganismsallow immunofluorescence

to act as a detection and screening tool in the monitoring of a

variety of materials. Foe example, research to adapt immuno-

fluorescence to food monitoring is an active field. In the pres-

ent, immunofluorescence provides the means by which

organisms can be sorted using the technique of flow cytome-

try. As individual bacteria, for example, pass by a detector, the

presence of fluorescence will register and cause the bacterium

to be shuttled to a special collection reservoir. Thus, bacteria

with a certain surface factor can be separated from the other

bacteria in the population that do not possess the factor

See alsoFluorescent dyes; Microscopy

IImmumogeneticsMMUNOGENETICS

Immunogenetics is the study of the mechanisms of autoim-

mune diseases, tolerance in organ transplantation, and immu-

nityto infectious diseases—with a special emphasis on the

role of the genetic make-up of an organism in these processes.

The immune systemevolved essentially to protect vertebrates

from a myriad species of potentially harmful infectious agents

such as bacteria, virus, fungiand various eukaryotic parasites.

However, the growing understanding of the immune system

has influenced a variety of different biomedical disciplines,

and is playing an increasingly important role in the study and

treatment of many human diseases such as cancer and autoim-

mune conditions.

There are two broad types of immune systems. The

innate immune system of defense depends on invariant recep-

tors that recognize common features of pathogens, but are not

varied enough to recognize all types of pathogens, or specific

enough to act effectively against re-infection by the same

pathogen. Although effective, this system lacks both speci-

ficity and the ability to acquire better receptors to deal with the

same infectious challenge in the future, a phenomenon called

immunological memory. These two properties, specificity and

memory, are the main characteristics of the second type of

immune system, known as the specific or adaptive immune

system, which is based on antigenspecific receptors. Besides

these two families of different receptors that help in immune

recognition of foreign infectious agents, both the innate and

the adaptive immune systems rely on soluble mediators like

the different cytokinesand kemokines that allow the different

cells involved in an immune response to communicate with

each other. The major focus of immunogeneticists is the iden-

tification, characterization, and sequencing of genes coding

for the multiple receptors and mediators of immune responses.

Historically, the launch of immunogenetics could be

traced back to the demonstration of Mendelian inheritance of

the human ABO blood groups in 1910. The importance of this

group of molecules is still highlighted by their important in

blood transfusion and organ transplantation protocols. Major

developments that contributed to the emergence of immuno-

genetics as an independent discipline in immunologywere the

rediscovery of allograft reactions during the Second World

War and the formulation of an immunological theory of allo-

graft reaction as well as the formulation of the clonal selection

hypothesis by Burnett in 1959. This theory proposed that

clones of immunocompetent cells with unique receptors exist

prior to exposure to antigens, and only cells with specific

receptors are selected by antigen for subsequent activation.

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