Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY History of immunology

271

•

causes the air passages to become constricted rather than

dilated. This response occurs in an attempt to keep the offend-

ing allergenic particles from being inhaled. Unfortunately, this

also makes breathing difficult. An example of such an effect of

histamine occurs in asthma.

Histamine has also been shown to function as a neuro-

transmitter (a chemical that facilitates the transmission of

impulses from one neural cell to an adjacent neural cell).

In cases of an extreme allergic reaction, adrenaline is

administered to eliminate histamine from the body. For minor

allergic reactions, symptoms can sometimes be lessened by

the use of antihistamines that block the binding of histamine

to a receptor molecule.

See alsoImmune system

HHistocompatibilityISTOCOMPATIBILITY

Histocompatibility refers to the means by which a eukaryotic

cell can be identified. The phenomenon is the result of the

presence of proteins on the surface of cells. These proteins are

referred to as histocompatibility molecules. The histocompat-

ibility molecules on the cells of one individual of a species are

unique. Thus, if the cell is transplanted into another person,

the cell will be recognized by the immune systemas being for-

eign. The histocompatibility molecules act as an antigen in the

recipient, and so can also be called a histocompatibility anti-

gen or transplantation antigen. This is the basis of the rejection

of transplanted material.

Identical twins have the same histocompatibility mole-

cules on their cells. Thus, tissue can be successfully trans-

planted from one individual to the other, because the tissue

will essentially not be foreign. However, for unrelated indi-

viduals, cells will have their own signature chemistry with

respect to the histocompatibility molecules. Tissue from one

individual will be recognized as foreign in another individual.

The suite of histocompatibility molecules present on the

surface of a cell is also referred to as the histocompatibility

complex. There are two classes of these molecules. The first

class is called class I molecules. These molecules are made up

of a portion that is embedded in the cell membrane and a por-

tion that protrudes out from the membrane’s outer surface. The

protruding portion is composed of both protein and sugar (car-

bohydrate). Some of the human leukocyte antigens are exam-

ples of class I molecules.

The class I molecules function to chemically tag a cell

so that the cell will be recognized and categorized by the T

lymphocyte cells of the immune system. The T cell will rec-

ognize a region of the histocompatibility complex as “self.”

Because of this recognition, there will not be an immune

response initiated against the cell. But, in another host, where

the same region is chemically different from class I groups on

the host cells, the introduced cells would be recognized as for-

eign by the T lymphocytes.

Another class of histocompatibility molecules called

class II are anchored into the cell membrane by have two seg-

ments of the molecule. At the outer surface of the cell the mol-

ecule contains an antigen that has been acquired from the sur-

rounding environment when particles were taken in and

degraded by host processes. This is called antigen presentation.

Class II molecules are on the surface of macrophages

and B-lymphocytes. These immune cells function to process

cells and present the antigens from these cells to T lympho-

cytes. This is done to increase the repertoire of antibodies that

an organism possesses. Antigen presentation of histocompati-

bility molecules “primes” the immune system. When an

invading organism is detected, the immune response can occur

much more swiftly than if no exposure to the antigen had ever

occurred.

The role of the histocompatibility complexes in immune

recognition of “self ” and “non-self” is the reason why trans-

plants are typically accompanied by the administration of drugs

that dampen down the immune response of the host. Only by

nullifying the host recognition of the class I and class II histo-

compatibility complexes can the transplant be maintained.

The genes that encode the histocompatibility determi-

nants are clustered together on the chromosome. These clus-

ters are referred to as the major and minor histocompatibility

complexes. The major compatibility genes are clustered

together on one chromosome. The minor compatibility genes

are located in several clusters throughout the genome.

Studies on mice, which also possess the histocompati-

bility complexes, have demonstrated that these complexes not

only play a role in transplant rejection, but also function in the

immune response to a variety of diseases. Mice that are genet-

ically different for a given histocompatibility complex will

respond differently to the same antigen. If a “non-self” histo-

compatibility complex is poorly recognized by the host

immune system, then an inadequate immune response will

ensue. The result can be the establishment of an infection.

See also Antibody and antigen; Immune system; Immu-

nodeficiency diseases; Major histocompatibility complex

(MHC)

HHistory of immunologyISTORY OF IMMUNOLOGY

In Western society, it was not until the late eighteenth century

that a rational approach to the origin of disease developed.

Prior to the discovery that disease was the result of pathogenic

organisms, it was commonly accepted that disease was a pun-

ishment from God (or the Gods), or even a witches curse.

Eastern cultures perceived disease as an imbalance in the

energy channels within the body. Later, the great plagues of

Europe were assumed the result of virulent or noxious vapors.

Nevertheless, there were intimations as early as 430 B.C. that

if one survived a disease, the person thereafter became

“immune” to any subsequent exposures. However, this was

never recognized as evidence of some type of internal defense

system until the later part of the seventeenth century.

Although most historical accounts credit Edward Jenner

for the development of the first immunizationprocess, a pre-

vious similar procedure had become established in China by

1700. The technique was called variolation. This was derived

womi_H 5/6/03 3:20 PM Page 271