Microbiology and Immunology

Immune stimulation, as a vaccine WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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test methods are not yet uniformly standardized across the

medical community. But, if the results from several tests are

positive for immune complexes, the validity of the diagnosis

is ensured.

Immune complex tests include the Raji cell, C1q bind-

ing, conglutinin, and anti-C3 assays. The Raji cell assay, for

example, detects the immune complexes following the binding

of the complexes with an immune molecule called comple-

ment. In addition, the complement has been labeled with a

compound known as fluorescein isothiocyanate. The latter

compound is able to fluoresce when light of a certain wave-

length is shone on it. The detection occurs in a machine called

a flow cytometer, in which fluid moves past a detector that is

programmed to detect certain chemical aspects. In the Raji cell

assay, detection of the fluorescent isothiocyanate indicates the

presence of the immune complex.

A normal result in an immune complex test is a negative

result. In other words, immune complexes are normally

absent.

See alsoAntibody-antigen, biochemical and molecular reac-

tions; Laboratory techniques in immunology

IImmune stimulation, as a vaccineMMUNE STIMULATION, AS A VACCINE

Immune stimulation refers to the stimulation of the immune

systemby an external source. The stimulation can confer a

protective effect against microorganisms. As well, immune

stimulation shows promise as a means of obtaining an immune

response to conditions such as cancer.

Conventionally, the immune system is stimulated into

producing antibodies or other infection-fighting constituents

in response to an infection. Immune stimulation seeks to elicit

the immune reaction before infection or other malady strikes,

as a means of preventing the infection or malady. This

approach is analogous to the administration of components of

weakened or inactive influenzavirus to protect people from

the subsequent onset of influenza.

The roots of the use of immune stimulation as a vaccine

date back to the late nineteenth century. Then, William Coley,

a New York bone surgeon, began treatments in which he

injected cancer sufferers with a preparation consisting of dis-

solved Streptococcus pyogenesbacteria. Anecdotal evidence

claimed remission of tumor growth in 40% of the treated

patients. Then, in the 1980s, it was discovered that the

observed anti-tumor activity of a bacteria known as Bacillus

Calmette-Guerinwas a property of the construction of the

bacterial genetic material. Indeed, the bacterial genetic mate-

rial is able to stimulate the immune system such that the target

sequence of the bacterial geneis distinguished from the host

genetic material. The resulting immune stimulation boosts

antibodylevels as well as another aspect of the immune sys-

tem known as cell-mediated immunity.

Synthetic peptides have also proved useful as agents of

immune stimulation. These compounds are made up of chains

of amino acids. They are called synthetic because they are not

naturally occurring, but rather are constructed in the labora-

tory. The peptide can contain amino acids in which a chemical

group is oriented in a mirror image of that which is normally

found in nature.

The mirror image arrangement proves lethal to various

bacteria. For example, synthetic peptides swiftly kill popula-

tions of Staphylococcus aureusand Enterococcus faeciumthat

are resistant to an array of antibiotics. The peptide binds to the

outer surface of the bacterium and is able to punch a hole

through the cell wall. The punctured bacteria die.

Furthermore, as the bacteria release their contents, the

immune system is stimulated to produce antibodies to the bac-

terial constituents. The synthetic peptide both kills bacteria

directly and stimulates an immune response that acts to kill

more bacteria.

Thus far, immune stimulation as a vaccine has been

developed towards so-called extracellular infections. These

are infection caused by bacteria that adhere to host cells or that

proliferate in fluids such as blood. For these types of infec-

tions, the immune stimulation aims to produce antibodies.

Defense against intracellular infections, which are caused by

bacteria invading host cells, requires the stimulation of other

immune components, such as phagocytic cells. Furthermore,

defense against viral infections requires stimulation of

immune components called killer cells.

Synthetic peptides can also stimulate the immune sys-

tem to recognize a surface constituent of a certain type of can-

cer cell called a melanoma cell. Melanoma is also commonly

referred to as skin cancer. The synthetic peptide can mimic the

peptide produced on a tumor that the immune system can rec-

ognize and respond to. By supplying the target externally, the

immune system has more opportunity to mount a defense

against the offending peptide. The resulting antibody mole-

cules would target the peptide on the tumor cells.

While shrinkage of tumors was evident in laboratories

studies, confirmation of the clinical power of the technique

requires a clinical trial where many people are given the treat-

ment and their progress monitored. Nonetheless, it has been

demonstrated that synthetic peptides are capable of stimulat-

ing the ability of the immune system to distinguish between

antigens that are an innate part of the body from those antigen

that come from outside of the body. In the case of many can-

cers, such immune stimulation is required, as the disease can

compromise the natural immune defenses.

See alsoImmunization; Immunologic therapies

IImmune synapseMMUNE SYNAPSE

Before they can help other immune cells respond to a foreign

protein or pathogenic organism, helper T cellsmust first

become activated. This process occurs when an antigen-pre-

senting cell submits a fragment of a foreign protein, bound to

a Class II MHCmolecule (virus-derived fragments are bound

to Class I MHC molecules) to the helper T cell. Antigen-pre-

senting cells are derived from bone marrow, and include both

dendritic cells and Langerhans cells, as well as other special-

ized cells. Because T cell responses depend upon direct con-

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