Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Severe combined immunodeficiency (SCID)

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basis of serology is the recognition of an antigenby immune

mechanisms, with the subsequent production of an antibody.

In medical terminology, serology refers to a blood test

to detect the presence of antibodies against a microorganism.

The detection of antibodies can be qualitative (i.e., determin-

ing whether the antibodies are present) or quantitative (i.e.,

determining the quantity of an antibody produced). Some

microorganismscan stimulate the production of antibodies

that persist in a person’s blood for a long time. Thus, in a qual-

itative assay the detection of a particular antibody does not

mean that the person has a current infection. However, it does

mean that it is likely that at some time that person was infected

with the particular microbial pathogen. Serology assays can be

performed at various times and the level of antibody deter-

mined. If the antibody level rises, it usually is indicative of a

response to an infection. The body produces elevated amounts

of the antibody to help fight the challenging antigen.

Serology as a science began in 1901. Austrian American

immunologist Karl Landsteiner(1868–1943) identified groups

of red blood cells as A, B, and O. From that discovery came

the recognition that cells of all types, including blood cells,

cells of the body, and microorganisms carry proteins and other

molecules on their surface that are recognized by cells of the

immune system. There can be many different antigens on the

surface of a microorganism, with many different antibodies

being produced.

When the antigen and the antibody are in suspension

together, they react together. The reaction can be a visible one,

such as the formation of a precipitate made up of a complex of

the antigen and the antibody. Other serology techniques are

agglutination, complement-fixation and the detection of an

antigen by the use of antibodies that have been complexed

with a fluorescent compound.

Serological techniques are used in basic research, for

example, to decipher the response of immune systems and to

detect the presence of a specific target molecule. In the clini-

cal setting, serology is used to confirm infections and to type

the blood from a patient. Serology has also proven to be very

useful in the area of forensics, where blood typing can be vital

to establishing the guilt or innocence of a suspect, or the iden-

tity of a victim.

See alsoAntibody and antigen; Antibody formation and kinet-

ics; Antibody-antigen, biochemical and molecular reactions;

Bacteria and bacterial infection; Immune system; Laboratory

techniques in immunology

SESSILE BACTERIA•seeBIOFILM FORMATION AND

DYNAMIC BEHAVIOR

SEVERE COMBINED IMMUNODEFICIENCY

(SCID)Severe combined immunodeficiency (SCID)

Severe combined immunodeficiency(SCID) is a rare genetic

disease that is actually a group of inherited disorders charac-

terized by a lack of immune response, usually occurring in

infants less than six months old. SCID is the result of a com-

bination of defects of both T-lymphocytesand B-lymphocytes.

Lymphocytes are white blood cells that are made in bone mar-

row, and many move to the thymus gland where they become

specialized immune T and B cells. In healthy individuals, T

cellsattack antigens while B cells make plasma cells that pro-

duce antibodies (immunoglobulins). However, this immune

response in SCID patients is absent making them very suscep-

tible to invading diseases, and thus children with untreated

SCID rarely live to the age of two years.

SCID is characterized by three main features. The

helper T-lymphocytes are functioning poorly or are absent, the

thymus gland may be small and functioning poorly or is

absent, and the stem cells in bone marrow, from which mature

T- and B-lymphocytes arise, are absent or defective in their

function. In all of these situations, little or no antibodies are

produced. If, for example, T-lymphocytes are never fully

developed, then the immune systemcan never function nor-

mally. Moreover, the results of these defects include the fol-

lowing: impairment of normal functioning T- and

B-lymphocytes, negative effects on the maturation process for

T-helper and T-suppressor cells, and elimination and damage

of the original source of the lymphocytes.

The immune disorders characterized in SCID arise

because of the inheritance of abnormal genes from one or both

parents. The most common form of SCID is linked to the X

chromosome inherited from the mother; this makes SCID more

common among males. The second most common defect is

caused by the inheritance of both parents’ abnormally inactive

genes governing the production of a particular enzyme that is

needed for the development of immunity, called adenosine

deaminase (ADA). Although many defective genes for other

forms of SCID have been identified in the last few years, sci-

entists do not fully understand all of the forms of the disease.

There are many specific clinical signs that are associ-

ated with SCID. After birth, an infant with SCID is initially

protected by the temporarily active maternal immune cells;

however, as the child ages, his or her immune system fails to

take over as the maternal cells become inactive. Pulmonary

problems such as pneumonia, non-productive coughs, inflam-

mationaround the bronchial tubes, and low alveolar oxygen

levels can affect the diseased infant repetitively. Chronic diar-

rhea is not uncommon, and can lead to severe weight loss,

malnutrition, and other gastrointestinal problem. Infants with

the disease have an unusual number of bacterial, fungal, viral,

or protozoal infections that are much more resistant to treat-

ment than in healthy children. Mouth thrushand yeastinfec-

tions, both fungal, appear in SCID patients and are very

resistant to treatment. Additionally, chronic bacterial and fun-

gal skin infectionsand several abnormalities of the blood cells

can persist.

Severe combined immunodeficiency is a disease that

can be successfully treated if it is identified early. The most

effective treatment has been hematopoietic stem cell trans-

plants that are best done with the bone marrow of a sister or

brother; however, the parent’s marrow is acceptable if the

infant is less than three months old. Early treatment can also

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