Bma Illustrated Medical Dictionary

Donated blood is tested for a range of

infectious agents such as hepatitis B

and hepatitis Cand antibodies to HIV.

After being classified into blood groups,

the blood is stored in a blood bank,

either whole or separated into its differ-

ent components (see blood products).

Apheresis is a type of blood donation in

which only a specific blood component,

such as plasma, platelets, or white

cells, is withdrawn from the donor.

blood filmA test that involves smear-

ing a drop of blood on to a glass slide

for examination under a microscope.

The blood film is stained with dyes to

make the blood cells show up clearly.

The test allows the shape and appear-

ance of blood cells to be checked for

any abnormality, such as the sickle-

shaped red blood cells characteristic of

sickle cell anaemia. The relative propor-

tions of the different types of white

blood cells can also be counted. This

examination, called a differential white

cell count, may be helpful in diagnosing

infection or leukaemia. Blood films are

also used in diagnosing infections, such

as malaria, in which the parasites can

be seen inside the red blood cells.

Blood films are usually carried out

together with a full blood count.

blood gasesA test for determining the

acidity–alkalinity (pH) and the concen-

trations of oxygen, carbon dioxide, and

bicarbonate in the blood. The test is

carried out on a sample of blood taken

from an artery, usually at the wrist or

groin. It is useful in diagnosing and mon-

itoring respiratory failure. Bicarbonate

and acidity reflect the acid–base balance

of the body, which may be disturbed in

conditions such as diabetic ketoacido-

sis, aspirin poisoning, hyperventilation,

or repeated vomiting. Blood oxygen can

also be measured without taking a

blood sample by using an oximeter.

blood glucose The level of glucosein

the blood. Abnormally high blood glu-

cose (sometimes called sugar) levels are

an indication of diabetes mellitus. (See

also hyperglycaemia; hypoglycaemia.)

blood groupsSystems of classifying

blood according to the different marker

proteins (antigens) on the surface of red

blood cells and antibodies in the plas-

ma. These antigens affect the ability of

the red blood cells to provoke an

immune response. There are 2 main

blood grouping systems: the ABO sys-

tem and the rhesus system.

In the ABO system, the presence or

absence of 2 types of antigen (named A

and B) on the surface of the red blood

cells determine whether a person’s

blood group is A, B, AB (which has both

A and B), or O (which has neither A nor

B). People with the A antigen (group A)

have anti-B antibodies; people with the

B antigen (group B) have anti-A anti-

bodies; those with both antigens (blood

group AB) have neither; and those with

neither antigen (group O) have both.

The rhesus system involves several

antigens, the most important of which

is factor D. People with this factor are Rh

positive; those without it are Rh neg-

ative. The importance of the Rh group

relates mainly to pregnancy in Rh-

negative women, since, if the baby

is Rh positive, the mother may form

antibodies against the baby’s blood

(see rhesus incompatibility).

BLOOD FILM BLOOD GROUPS

B

BLOOD GROUP AB

BLOOD GROUPS

B antigen

Anti-B

antibody Anti-A antibody

Anti-A

antibody

B antigen

A antigen

A antigen

Anti-B

antibody

BLOOD GROUP A BLOOD GROUP B BLOOD GROUP O

Red blood

cell