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Summary Threats and defence mechanisms BBS2001
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THREATS AND DEFENCEMECHANISMS BBS2001
, Case 01 BBS2001
WHAT IS COMPOSITION OF BLOOD AND WHAT IS ITS FUNCTION ?
Composition:
Blood is a specialized connective tissue in which living blood cells, called the formed elements, are
suspended in a non-living fluid matrix called plasma.
Blood lacks the collagen and elastic fibres typical of other connective tissues, but dissolved fibrous
proteins become visible as fibrin strands during blood clotting.
In a tube that is spun most of the reddish mass at the bottom of the tube is erythrocytes which are
the red blood cells that transport oxygen. A thin, whitish layer called the buffy coat is present and
contains leukocytes, the white blood cells that act in various ways to protect the body, and platelets,
cell fragments that help stop bleeding. Erythrocytes normally constitute about 45% of the total
volume of a blood sample, a percentage which is known as the haematocrit. Leukocytes and
platelets contribute less than 1% of blood volume. Plasma makes up most of the remaining 55% of
whole blood.
WHAT ARE THE DIFFERENT TYPE OF BLOOD CELLS AND WHAT IS THEIR FUNCTION (FORMED ELEMENTS )?
Erythrocytes have no nuclei or organelles, and
platelets are cell fragments. Only leukocytes are
complete cells. Most of the formed elements
survive in the bloodstream for only a few days.
Most blood cells do not divide. Instead, stem
cells divide continuously in red bone marrow to
replace them.
If you examine a stained smear of human blood
under the light microscope, you will see disc-
shaped red blood cells, a variety of gaudily
stained spherical white blood cells, and some
scattered platelets that look like debris.
1
,WHITE BLOOD CELLS
Leukocytes or white blood cells (WBCs), are the only formed elements that are complete cells, with
nuclei and the usual organelles. Accounting for less than 1% of total blood volume, leukocytes are far
less numerous than red blood cells. On average, there are 4800-10,800 WBCs/pl of blood. Leukocytes
are crucial to our defence against disease. They form a mobile army that helps protect the body from
damage by bacteria, viruses, parasites, toxins, and tumour cells. As such, they have special functional
characteristics. Red blood cells are confined to the bloodstream, and they carry out their functions in
the blood. But white blood cells are able to slip out of the capillary blood vessels—a process called
diapedesis. The circulatory system is simply their means of transport to areas of the body (mostly
loose connective tissues or lymphoid tissues) where they mount inflammatory or immune responses.
Leukocytes are grouped into two major categories on the basis of structural and chemical
characteristics:
1. Granulocytes
a. Contain obvious membrane-bound cytoplasmic granules:
b. Granulocytes are all roughly spherical in shape. They are larger and much shorter
lived than erythrocytes.
c. Lobed nuclei (rounded nuclear masses connected by thinner strands of nuclear
material), and their membrane-bound cytoplasmic granules stain quite specifically
with Wright’s stain. Functionally, all granulocytes are phagocytes to some degree.
i. Neutrophils
1. Neutrophils, the most numerous white blood cells, account for 50-
70% of the WBC population.
2. Neutrophils are about twice as large as erythrocytes.
3. The neutrophil cytoplasm contains very fine granules (of two
varieties) that are difficult to see.
4. Neutrophils get their name (literally, “neutral-loving”) because their
granules take up both basic (blue) and acidic (red) dyes.
5. Together, the two types of granules give the cytoplasm a lilac color.
6. Some of these granules contain hydrolytic enzymes, and are
regarded as lysosomes.
7. Others, especially the smaller granules, contain a potent “brew” of
antimicrobial proteins, called defensins. Neutrophil nuclei consist of
three to six lobes. Because of this nuclear variability, they are often
called polymorphonuclear leukocytes (PMNs) or simply polys
(polymorphonuclear = many shapes of the nucleus).
8. Neutrophils are our body’s bacteria slayers, and their numbers
increase explosively during acute bacterial infections such as
meningitis and appendicitis.
9. Neutrophils are chemically attracted to sites of inflammation and are
active phagocytes.
10. They are especially partial to bacteria and some fungi, and bacterial
killing is promoted by a process called a respiratory burst.
11. In the respiratory burst, the cells metabolize oxygen to produce
potent germ-killer oxidizing substances such as bleach and hydrogen
peroxide. In addition, defensin-mediated lysis occurs when the
granules containing defensins merge with a microbe containing
2
, phagosome. The defensins form peptide “spears” that pierce holes in
the membrane of the ingested “foe.”
12. Light grey to pink cytoplasm.
ii. Eosinophils
1. Eosinophils account for 2-4% of all leukocytes and are approximately
the size of neutrophils. Their nucleus usually resembles an old-
fashioned telephone receiver—it has two lobes connected by a
broad band of nuclear material.
2. Large, coarse granules that stain from brick red to crimson with acid
(eosin) dyes pack the cytoplasm. These granules are lysosome-like
and filled with a unique variety of digestive enzymes. However,
unlike typical lysosomes, they lack enzymes that specifically digest
bacteria.
3. The most important role of eosinophils is to lead the counterattack
against parasitic worms, such as flatworms (tapeworms and flukes)
and roundworms (pinworms and hookworms) that are too large to
be phagocytized. These worms are ingested in food (especially raw
fish) or invade the body via the skin and then typically burrow into
the intestinal or respiratory mucosae.
4. Eosinophils reside in the loose connective tissues at the same body
sites, and when they encounter a parasitic worm “prey,” they gather
around and release the enzymes from their
cytoplasmic granules onto the parasite’s
surface, digesting it away.
5. Eosinophils have complex roles in many other
diseases including allergies and asthma. While
they contribute to the tissue damage that
occurs in many immune processes, we are
also beginning to recognize them as
important modulators of the immune
response.
6. Granules are coarse/bright red to honey
yellow.
7. Densely packed granules.
8. Phagocytosis
3
, iii. Basophils
1. Basophils are the rarest white blood cells, accounting for only 0.5-1%
of the leukocyte population.
2. Releasing content of chemicals
3. Their cytoplasm contains large, coarse, histamine-containing
granules that have an affinity for the basic dyes (basophil = base
loving) and stain purplish-blue.
4. Histamine is an inflammatory chemical that acts as a vasodilator
(makes blood vessels dilate) and attracts other
white blood cells to the inflamed site; drugs
called antihistamines counter this effect.
5. The deep purple nucleus is generally U or S
shaped with one or two conspicuous
constrictions.
6. Granulated cells similar to basophils, called mast
cells, are found in connective tissues. Although
mast cell nuclei tend to be more oval than
lobed, the cells are similar microscopically, and
both cell types bind to a particular antibody
(immunoglobulin E) that causes the cells to
release histamine. However, they arise from
different cell lines.
2. Agranulocytes
a. Lack obvious granules
i. Lymphocytes
1. Lymphocytes, accounting for 25% or more of the WBC population,
are the second most numerous leukocytes in the blood. When
stained, a typical lymphocyte has a large, dark-purple nucleus that
occupies most of the cell volume.
2. The nucleus is usually spherical but may be slightly indented, and it is
surrounded by a thin rim of pale-blue cytoplasm. Lymphocyte
diameter ranges from 5 to 17 pm, but they are often classified
according to size as small (5-8 pm), medium (10-12 pm), and large
(14-17 pm). Large numbers of lymphocytes exist in the body, but
relatively few (mostly the small lymphocytes) are found in the
bloodstream.
3. In fact, lymphocytes are so called because most are closely
associated with lymphoid tissues (lymph nodes, spleen, etc.), where
they play a crucial role in immunity. T lymphocytes (T cells) function
in the immune response by acting directly against virus-infected cells
and tumour cells. B lymphocytes (B cells) give rise to plasma cells,
which produce antibodies.
ii. Monocytes
1. Monocytes account for 3-8% of WBCs. With an average diameter of
18 pm, they are the largest leukocytes. They have abundant pale-
blue cytoplasm and a darkly staining purple nucleus, which is
distinctively U or kidney shaped.
4
, 2. When circulating monocytes leave the bloodstream and enter the
tissues, they differentiate into highly mobile macrophages with
prodigious appetites. Macrophages are actively phagocytic, and they
are crucial in the body’s defence against viruses, certain intracellular
bacterial parasites, and chronic infections such as tuberculosis.
Leukocytes
1. Myeloid:
a. Monocytes: these are important in acute and chronic infections. They actively
phagocytize and are important components of cell-mediated immunity. When
monocytes go to the tissue, they are referred to as macrophages. These are more
effective than neutrophils, but they arrive later (within 12 hours).
b. Dendritic derive from monocytes which respond to microbes by producing cytokines
that recruit leukocytes and initiate adaptive immune responses.
i. Presenting antigens
2. Lymphocytes:
a. These participate in cellular and humoral defence against infections. Lymphocytes
can also be subcategorized:
▪ T-Lymphocyte: these can attack the invaders from inside the cells.
• T helper
▪ B-lymphocyte: these can attack the invaders from outside the cells.
b. Natural killer cells, also known as NK cells or large granular lymphocytes (LGL), are a
type of cytotoxic lymphocyte critical to the innate immune system.
Production and life Span of leukocytes
Like erythropoiesis, leukopoiesis, or the production of white blood cells, is stimulated by chemical
messengers. These messengers, which can act either as paracrine or hormones, are glycoproteins
that fall into two families of hematopoietic factors, interleukins and colony-stimulating factors, or
CSFs. The interleukins are numbered (e.g., 1L-3, IL-5), but most CSFs are named for the leukocyte
population they stimulate—for example, granulocyte-CSF (G-CSF) stimulates production of
granulocytes. Hematopoietic factors, released by supporting cells of the red bone marrow and
mature WBCs, not only prompt the white blood cell precursors to divide and mature, but also
enhance the protective potency of mature leukocytes.
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