Blood

Blood is the medium by which substances or materials are transported. Blood is actually a tissue.
It is thick because it is made up of a variety of cells, each having a different job. It is
made up of about:
• 55% blood plasma;
• 45% blood cells.
The blood plasma is about 90% water and most of the substances which must be transported are
dissolved in it. This includes dissolved food (glucose, amino acids, fatty acids and glycerol),
carbon dioxide (as the bicarbonate ion), nitrogenous wastes, hormones and mineral salts (as ions
such as Na+, K+, Cl+). The blood cells are of two main types, red and white. There are also
fragments of cells called platelets.

Components of blood

Comparison between the solid and liquid part of blood

Your blood is made up of liquid and solids. The liquid part, called plasma, is made of water,
salts, and protein. Over half of your blood is plasma. The solid part of your blood contains red
blood cells, white blood cells, and platelets.
Structure and function of types of blood cells
There are three types of blood cells. They are:
1.Red blood cells (Erythrocytes)
2.White blood cells (Leukocytes)
3.Platelets (Thrombocytes)

1) Platelets help the blood to clot. Clotting stops the blood from flowing out of the body
when a vein or artery is broken. Platelets are also called thrombocytes.

2) Red blood cells carry oxygen. Of the 3 types of blood cells, red blood cells are the most
plentiful. In fact, a healthy adult has about 35 trillion of them. The body creates these
cells at a rate of about 2.4 million a second, and they each have a life span of about 120
days. Red blood cells are also called erythrocytes.
3) White blood cells ward off infection. These cells, which come in many shapes and sizes,
are vital to the immune system. When the body is fighting off infection, it makes them in
ever-increasing numbers. Still, compared to the number of red blood cells in the body, the
number of white blood cells is low. White blood cells are also called leukocytes.

There are several types of white blood cells which includes:

- Neutrophil- The most common type of white blood cell is the neutrophil, which is
the “immediate response” cell and accounts for 55 to 70 percent of the total white
blood cell count. Each neutrophil does live for less than a day, so your bone marrow
must constantly make new neutrophils to maintain protection against infection.
Transfusion of neutrophils is generally not effective since they do not remain in the
body for very long.

- Phagocytes- Phagocytes are a type of white blood cell that use phagocytosis to engulf
bacteria, foreign particles, and dying cells to protect the body. They bind to pathogens
and internalize them in a phagosome, which acidifies and fuses with lysosomes in
order to destroy the contents.

- Lymphocytes- The other major type of white blood cell is a lymphocyte. There are
two main populations of these cells. T lymphocytes help regulate the function of other
immune cells and directly attack various infected cells and tumors. B lymphocytes
make antibodies, which are proteins that specifically target bacteria, viruses, and
other foreign materials.
Blood clotting
When the skin is cut and a small blood vessel is broken, a blood clot forms to prevent further
blood loss. A series of reactions take place at the site of the cut vessel which results in the
formation of fibrin, an insoluble fibrous protein which traps blood cells and plugs the gap. The
clot also prevents the entry of disease-causing organisms. Loss of blood from a vessel is called a
hemorrhage, and losing a lot of blood could result in death. In this case, a blood transfusion can
be given to replace blood and save the person’s life.

Blood types/Blood groups

There are 4 different blood types: A, B, AB, and O. Genes that you inherit from your parents (1
from your mother and 1 from your father) determine your blood type.
Blood is always being made by the cells inside your bones, so your body can usually replace any
blood lost through small cuts or wounds. But when a lot of blood is lost through large wounds, it
has to be replaced through a blood transfusion (blood donated by other people). In blood
transfusions, the donor and recipient blood types must be compatible. People with type O blood
are called universal donors, because they can donate blood to anyone, but they can only receive a
transfusion from other people with type O blood.
Functions of blood
Blood carries oxygen from the lungs and nutrients from the digestive tract to the body’s cells. It
also carries away carbon dioxide and all of the waste products that the body does not need. (The
kidneys filter and clean the blood.) Blood also:
• Helps keep your body at the right temperature
• Carries hormones to the body’s cells
• Sends antibodies to fight infection
• Contains clotting factors to help the blood to clot and the body’s tissues to heal

Immunity- Immunity is defined as the body’s ability to protect itself from an infectious disease.
When you are immune to a disease, your immune system can fight off infection from it.
Antibody - An antibody is a Y- shaped protein component of the immune system that circulates
in the blood, recognizes foreign substances like bacteria and viruses, and neutralizes (having
neither acidic nor basic properties) them. After exposure to a foreign substance, called an
antigen, antibodies continue to circulate in the blood, providing protection against future
exposures to that antigen.
Antigens and antibodies
Anything that is foreign or different and causes antibody formation is called an antigen.When
antigens, such as the measles virus, enter the body, lymphocytes recognise them and start to
produce specific antibodies on a large scale to destroy the viruses. The immune response is very
specific- only the antibodies for that particular antigen are made. To defend the body against
disease, antibodies act in a number of ways:
• they cause the antigens to clump together resulting in their death and easy removal by the
phagocytes;
• they neutralise toxins produced by the antigens;
• they prevent the antigen from entering body cells.
Recognition of antigens and production of the specific antibodies against them takes time.
During that time, the antigens will have produced symptoms of the disease. Once the antibodies
are produced, the antigens or the toxins they produce are destroyed or neutralised and the
symptoms disappear. The antibodies then gradually disappear from the blood, but they leave
behind special memory lymphocytes. If the specific antigen invades a second time, the memory
lymphocytes immediately recognise them, and rapidly make large amounts of the specific
antibody. This time, the antigens are destroyed before symptoms develop, and the person is said
to be immune to that disease. This happens naturally and is called natural immunity.

Immunity is a rapid large increase of antibodies in the blood. A rise in body temperature, or
fever, can happen with some infections. This is actually an immune system response. A rise in
temperature can kill some microbes. Fever also triggers the body’s repair process.
There are two types of natural immunity.
- Actively acquired immunity- When the body has already experienced an infection
by a pathogen or antigen, as described above, the lymphocytes produce large
quantities of antibodies to fight the disease before symptoms develop a second time.

- Passively acquired immunity- Antibodies can pass across the placenta providing a
newborn baby with immunity against diseases that the mother’s body is immune to.
Also, antibodies present in breast milk help to protect the baby against antigens.
Artificial immunity
Artificial immunity is a mean by which the body is given immunity to a disease by intentional
exposure to small quantities of it.
Immunization provides immunity to communicable diseases which are also known as infectious
diseases. This is achieved by injecting, or administering orally, small amounts of dead or
weakened (attenuated) antigens into the body. This is called vaccination. The body is stimulated
to produce antibodies. One example is the MMR vaccine given at around 2 years of age or
younger to protect children against measles, mumps and rubella (viral infection that causes red
rash). DTP vaccines, administered at any age, protect against diphtheria (caused by a bacteria
that makes toxins to lead to difficulty in breathing, heart failure and even death), tetanus (caused
by a bacteria that produces toxin and effects the brain and nervous system) and pertussis
(whooping cough). Smallpox has been eradicated (destroyed completely) because of
immunization programs. Vaccines against tuberculosis (TB) and hepatitis B (infection of the
liver) have also been developed, but there are still not vaccines against diseases such as cancers,
leprosy, malaria and AIDS, despite much research.
There are two types of artificial immunity:
- Actively acquired - This is by vaccination at a suitable time in the person’s life,
when they are not infected with the antigen. The vaccine used contains treated
antigens which cannot cause the disease, but which can stimulate the body to make
antibodies. Immunity is obtained because if the real antigen should enter the body,
antibodies are immediately and rapidly produced to destroy it. This happens before
symptoms develop and the person is said to be immune to that disease.

- Passively acquired - The vaccine contains ready-made antibodies which provide

immediate relief by destroying the antigens. This is given when the person has been
infected with the antigens and has no previous immunity.
References:
1) Alberts, Bruce; Johnson, Alexander; Lewis, Julian; Raff, Martin; Roberts, Keith; Walter,
Peter (2002). Molecular Biology of the Cell (4th ed.). New York: Garland Science.

2) Barclay AN (August 2003). “Membrane proteins with immunoglobulin-like domains-a

master superfamily of interaction molecules”. Seminars in Immunology. 15 (4): 215-23.

3) Lindenmann J (April 1984). “Origin of the terms ‘antibody’ and ‘antigen’”. Scandinavian
Journal of Immunology. 19 (4): 281-5.