Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Immunology

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ematical models. Immunoblotting is usually performed in the

form of Western blotting, which is reserved to the detection of

proteins and involves an electrophoresisseparation step fol-

lowed by electroblotting of the separated proteins from the gel

to a membrane and then probing with an antibody. Detection

of the antigen protein antibody interaction is made in a similar

way as in RIA or ELISA depending on whether a radiolabeled

or enzyme-coupled antibody is used.

Antibodies can also be monitored through immunoas-

says that are based on the ability of antibodies to alter the

physical state of their corresponding antigens and typically

involve the creation of a precipitate in a solid or liquid

medium. The hemmaglutination assay used to determine the

ABO type of blood groups and match compatible donors and

recipients for blood transfusion is based on this assay.

Currently, the most common application of this immunoassay

is in a procedure known as immunoprecipitation. This method

allows antibodies to form complexes with their antigen in a

complex mixture like the cytosol, the nucleusor membrane

complexes of the cell. The antigen-antibody complex is pre-

cipitated either by inducing the formation of even larger com-

plexes through the addition of excess amounts of

anti-immunoglobulin antibodies or by the addition of agarose

beads coupled to a special class of bacterial proteins that bind

the Fc region of the antibody. The complex can also be pre-

cipitated by covalently linking the antibody to agarose beads

forming a special affinity matrix. This procedure will also

allow the purification of the antigen by immunoaffinity, a spe-

cial form of affinity chromatography. Immunoprecipitation is

a valuable technique that led to major discoveries in immunol-

ogy an all disciplines of molecular and cellular biology. It

allows the precipitation of the antigen in complex with other

interacting proteins and reagents and therefore gives an idea

on the function of the antigen.

The T cell immune response is detected by using mon-

oclonal antibodies, a specific family of antibodies that recog-

nize surface markers that are expressed by lymphocytes upon

their activation. These monoclonal antibodies are highly spe-

cific, and are produced by special techniques from single

clones of B cellsand are therefore, homogenous groups of

immunoglobulins with the same isotype and antigen binding

affinity. These antibodies are used to identify characterize

cells by flow cytometry (FACS), immunocytochemistry,

immunofluorescencetechniques. The difficulty to isolate anti-

gen specific T cells is due to the fact that these T cells recog-

nize the antigen in the context of a tri-molecular complex

involving the T cell receptor and the MHCmolecules on the

surface of specialized cells called antigen-presenting cells.

These interactions are subtle, have low affinity and are

extremely complex to study. Novel and powerful techniques

using tetramers of MHC molecules were developed in 1997

that are now used to identify and isolate antigen specific T cell

clones. These tetramer-based assays are proving useful in sep-

arating very rare cells, and could be used in clinical medicine.

In fact, virus and tumor specific T cells usually give a stronger

response and are usually more effective in killing virus

infected and tumor cells. Testing for the function of activated,

antigen specific T cells known as effector T cells is routinely

done in vitroby testing for cytokine production, cytotoxicity

to other cells and proliferation in response to antigen stimula-

tion. Local reactions in the skin of animals and humans pro-

vide information about T cell responses to an antigen, a

procedure that is very used in testing for allergic reactions and

the efficacy of vaccination procedures. Experimental manipu-

lations of the immune system in vivo are performed to reveal

the functions of each component of the immune system in

vivo.Mutationsthrough irradiation, or mutations produced by

genetargeting (e.g., knock-out and knock-in techniques), as

well as animal models produced by transgenic breeding, are

proving helpful to researchers in evaluating this highly com-

plex system.

See alsoImmune complex test; Immune stimulation, as a vac-

cine; Immune synapse; Immunity, active, passive and delayed;

Immunity, cell mediated; Immunity, humoral regulation;

Immunization; Immunochemistry; Immunodeficiency;

Immunoelectrophoresis; Immunofluorescence; Immunogene-

tics; Immunologic therapies; Immunology; Immunomodu-

lation; Immunosuppressant drugs; In vitro and in vivo

research; Laboratory techniques in immunology

IMMUNOLOGICAL ASPECTS OF REPRO-

DUCTION•seeREPRODUCTIVE IMMUNOLOGY

IImmunologyMMUNOLOGY

Immunology is the study of how the body responds to foreign

substances and fights off infection and other disease.

Immunologists study the molecules, cells, and organs of the

human body that participate in this response.

The beginnings of our understanding of immunitydate

to 1798, when the English physician Edward Jenner

(1749–1823) published a report that people could be protected

from deadly smallpoxby sticking them with a needle dipped

in the material from a cowpoxboil. The French biologist and

chemist Louis Pasteur(1822–1895) theorized that such immu-

nizationprotects people against disease by exposing them to a

version of a microbe that is harmless but is enough like the

disease-causing organism, or pathogen, that the immune sys-

temlearns to fight it. Modern vaccines against diseases such

as measles, polio, and chicken pox are based on this principle.

In the late nineteenth century, a scientific debate was

waged between the German physician Paul Ehrlich

(1854–1915) and the Russian zoologist Élie Metchnikoff

(1845–1916). Ehrlich and his followers believed that proteins

in the blood, called antibodies, eliminated pathogens by stick-

ing to them; this phenomenon became known as humoral

immunity. Metchnikoff and his students, on the other hand,

noted that certain white blood cells could engulf and digest

foreign materials: this cellular immunity, they claimed, was

the real way the body fought infection.

Modern immunologists have shown that both the

humoral and cellular responses play a role in fighting disease.

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