Medical Biology

Dr. Khalida Ibrahim

Blood
Granulocytes:
❖ Neutrophils:
They constitute about 60-70% of the circulating
leukocytes, their nucleus consists of 2-5 lobes (usually
3) linked together by fine chromatin thread. The
cytoplasm of the neutrophils is filled with specific
granules that are small in size.
These cells circulate in the blood in a resting state but
with appropriate activation they leave the blood and
enter the tissues where they become highly motile,
phagocytic cells and their primary function is to ingest
and destroy the invading organisms. Once neutrophils
perform their function of killing microorganisms they
die, resulting in the formation of pus, the accumulation of dead WBC with
bacteria and tissue fluid.

❖ Eosinophils:
Are less numerous than neutrophils, they form only
2-4% of total WBC count. They have a characteristic
bilobed nucleus. Their main feature is the presence of
many large & elongated granules that are stained by
eosin.
Number of eosinophils increase greatly in many types
of parasitic infestations & the protection against the
parasitic disease is one of their major functions. They
also increase in allergic states.

❖ Basophils:
Constitute less than 1% of the total WBC count, their
nucleus is divided into irregular lobes, but this division
is not so obvious. They have a cytoplasmic granule that
are large and intensely basophilic they are irregular in
size & shape contain histamine (vasodilator) and
heparin (anticoagulant). Basophils might be the
 precursors of tissue mast cells with which they have many structural &
functional similarities.

Agranulocytes:
❖ Lymphocytes:
Lymphocytes make up 20-25% of WBCs. They are groups of spherical cells
with similar morphological characters, they have an ovoid nucleus, the
cytoplasm of the cell is scanty and can be seen as a thin
rim around the nucleus. there are two main types of
lymphocytes: T lymphocytes & B lymphocytes and
those lymphocytes are responsible for immune
surveillance to detect any foreign particle in the tissues.
Activation of B lymphocytes after an immune response
to a foreign particle leads to their differentiation into
plasma cells. Those cells are large cells with eccentric
rounded nucleus, and they are responsible for active
synthesis of immunoglobulins. In health plasma cells are
not found in the blood but they are seen in small
population in lymphoid organs.

❖ Monocytes:
They are the largest of the leukocytes. They constitute 3-8% of the white
blood cells. They are spherical cells with oval or kidney shaped nucleus
which is often placed eccentrically. Their
cytoplasm is basophilic. Monocytes can
live in the blood for 8 hours, after which
they move into the connective tissue,
where they may remain for a few months
or longer. Blood monocytes are the
precursor cells of tissue macrophages and
other cells of the mononuclear phagocytic
system such as kupffer cells in the liver,
pulmonary and alveolar macrophages.

Clinical notes:
Increased numbers of white cells appear in the peripheral blood in a variety of
disorders and provide a useful clue to underlying disease. For example:
1. An increase of circulating neutrophils in bacterial infections (neutrophilia)
2. An increase of circulating eosinophil in parasitic infestations and some allergies
(eosinophilia)
3. An increase in circulating lymphocytes in certain viral infections
(lymphocytosis).
4. An increase in circulating basophils indicate presence of chronic inflammation.
5. Increase in monocyte levels indicate inflammation and infection.

Platelets (thrombocytes):
Are non-nucleated, small, disk like cells formed
from fragmentation of a giant cell in the bone marrow
called megakaryocytes. Platelets will promote blood
clotting & help in repairing gaps in the wall of blood
vessels. Their count is 150- 400 ×103/mm3. They have
a life span of only 10 days.

Disorders related to platelets:

There are lots of different groups of disorders affecting the platelets:
Thrombocythaemia – where there are too many platelets in circulation.
Thrombocytopenia – where there are too few platelets in circulation.
Dysfunction disorders – where there are the correct number of platelets in
circulation but they do not work properly.
All of these disorders mean that the clotting process is disrupted so lead to
abnormal clot formation and bleeding.
An insufficient number of platelets is called thrombocytopenia. Thrombocytopenia
is either due to low platelet production in bone marrow or increased breakdown of
platelets outside the marrow. A number of conditions, including leukemia, can
lead to thrombocytopenia. It can also be drug-induced. Symptoms include bruising,
rash, and nosebleeds or bleeding in the mouth. Gastrointestinal bleeding or
bleeding in the brain are possible complications.

Blood Clotting:
The blood-clotting process helps the body maintain homeostasis in the
cardiovascular system by ensuring that the plasma and formed elements remain
within the blood vessels. At least 12 clotting factors and calcium ions (Ca2+)
participate in the formation of a blood clot. Clot formation is initiated when a
blood vessel is damaged. At the site of the break in the blood vessel, platelets and
damaged tissue release prothrombin activator, which, with the assistance of
calcium ions, converts the plasma protein prothrombin (synthesized by the liver
in a vitamin K-dependent reaction and is released into the circulation) to thrombin.
Thrombin, in turn, acts as enzyme to cut two short amino acid chains from each
fibrinogen molecule. The fibrin fragments then join end to end, forming long
threads of fibrin. Fibrin threads wind around the platelet plug in the damaged area
of the blood vessel and provide the framework for the clot.
Red blood cells trapped within the fibrin threads make the clot appear red. A fibrin
clot is temporary. Once blood vessel repair starts, an enzyme called plasmin
destroys the fibrin network so tissue cells can grow. After blood clots, a yellowish
fluid called serum escapes from the clot. It contains all the components of plasma
except fibrinogen and prothrombin.
Disorders Related to Blood Clotting
Hemophilia A and B are clinically identical, differing only in the deficient factor.
Both are due to X-linked recessive inherited disorders. Blood from hemophiliac
patients does not coagulate normally: the blood clotting time is prolonged. Persons
with this disease bleed severely even after mild injuries, such as a skin cut, and
may bleed to death after more severe injuries. The blood plasma of patients with
hemophilia A is deficient in clotting factor VIII or contains a defective factor VIII,
one of the plasma proteins involved in fibrin generation; in hemophilia B
(Christmas Disease), the defect is in factor IX. In severe cases the blood is
incoagulable. There are spontaneous hemorrhages in body cavities, such as major
joints and the urinary tract. Generally, only males are affected by hemophilia A,
because the recessive gene to factor VIII is on the X chromosome. Females may
have one defective X chromosome, but the other one is usually normal. Females
develop hemophilia only when they have the abnormal gene in both X
chromosomes, a rare event. However, women with a defective X chromosome may
transmit the disease to their male children.