Microbiology and Immunology

Antigenic mimicry WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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from a single cell that produces a single antibody, then divides

many times. The cells that divided do not contain antibodies

and could, therefore, be crossed with lymphocytes to produce

specific antibodies. These hybrid cells are called hybridoma,

and they produce monoclonal antibodies.

One molecular biologist who needed pure antibodies for

a study of myeloma mutationswas the Argentinean César

Milstein(1927– ). After receiving a doctorate in biochemistry,

specializing in enzymes, from the University of Buenos Aires

in 1957, he continued this study at the University of

Cambridge in England. There he worked under the biochemist

Frederick Sanger and earned another doctorate in 1961.

Milstein had returned to Argentina, but political disturbances

forced him to flee the country. He came back to Cambridge,

where Sanger suggested that he work with antibodies.

In 1974, Milstein was working with Georges Köhler

(1946–1995), a German postdoctoral student who had just

received his doctorate from the University of Freiburg for

work performed at the Institute for Immunologyin Basel,

Switzerland. To produce the needed antibodies, Milstein and

Köhler first injected a mouse with a known antigen. After

extracting the resulting lymphocytes from the mouse’s blood,

they fused one of them with a myeloma cell. The resulting

hybrid produced the lymphocyte’s specific antibody and

reproduced endlessly. As Milstein soon realized, their tech-

nique for producing monoclonal antibodies could be used in

many capacities. Milstein and Köhler shared part of the 1984

Nobel prize in physiology or medicine for their invention.

Today pure antibodies are made using the Milstein-

Köhler technique and also through genetic engineering, which

adds the genefor the desired antibody to bacteria that can pro-

duce it in large amounts. Monoclonal antibodies are instru-

mental in the performance of sensitive medical diagnostic

tests such as: determining pregnancy with chorionic

gonadotropin; determining the amino acid content of sub-

stances; classifying antigens; purifying hormones; and modi-

fying infectious or toxic substances in the body. They are also

important in cancer treatment because they can be tagged with

radioisotopes to make images of tumors.

See alsoAntibody-antigen, biochemical and molecular reac-

tions; Antibody and antigen; Immunity, cell mediated;

Immunogenetics; Immunologic therapies; Immunological

analysis techniques; In vitroand in vivoresearch

ANTIGEN• seeANTIBODY AND ANTIGEN

AAntigenic mimicryNTIGENIC MIMICRY

Antigenic mimicry is the sharing of antigenic sites between

microorganismsand mammalian tissue. An immune response

can be directed both at the microorganism and at the host site

that shares the antigenic determinant. This autoimmune

response due to antigenic mimicry is known to be a crucial

factor in the development of certain ailments in humans.

The immune system recognizes three-dimensional

structure of protein. A protein, which is made up of a sequence

of amino acids strung together, will fold up in various ways,

depending on whether a section is more hydrophilic (“water

loving”) or hydrophobic(“water hating”), and depending on

the function of various regions of the protein.

Proteins that adopt a similar three-dimensional configu-

ration can stimulate a common response from the immune sys-

tem. Typically, proteins that have a similar amino acid

sequence will adopt the similar folded structures. For exam-

ple, the bacteriaChlamydia pneumoniae, Chlamydia psittaci,

and Chlamydia trachomatispossess a protein that is part of the

bacterial outer membrane. This protein is similar in amino

acid sequence to a portion of a protein, called alpha-myosin

heavy chain, which is found specifically in the heart muscle of

humans and animals such as mice. In mice, an immune reac-

tion to Chlamydia triggers a condition known as inflammatory

heart disease. A continued host autoimmune response damages

the heart, leading to cardiac malfunction. Indeed, it has been

shown that a significant number of patients with heart disease

have antibodies to Chlamydia in their blood, indicative of a

past infection with the bacteria.

Rheumatoid arthritis is another example of a malady

that is the consequence of an autoimmune reaction.

Mice used to develop the monoclonal cells that secret a specific

antibody.

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