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Learning objectives

  • Identify and describe the components of blood
  • Explain the process by which the formed elements of blood are formed (hematopoiesis)
  • Describe the characteristics of formed elements found in peripheral blood, as well as their respective functions within the innate immune system

In the previous section, we discussed some of the chemical mediators found in plasma, the fluid portion of blood. The nonfluid portion of blood consists of various types of formed elements , so called because they are all formed from the same stem cells found in bone marrow. The three major categories of formed elements are: red blood cells (RBCs), also called erythrocyte s ; platelets , also called thrombocytes ; and white blood cells (WBCs), also called leukocytes .

Red blood cells are primarily responsible for carrying oxygen to tissues. Platelets are cellular fragments that participate in blood clot formation and tissue repair. Several different types of WBCs participate in various nonspecific mechanisms of innate and adaptive immunity. In this section, we will focus primarily on the innate mechanisms of various types of WBCs.

Hematopoiesis

All of the formed elements of blood are derived from pluripotent hematopoietic stem cells (HSCs) in the bone marrow. As the HSCs make copies of themselves in the bone marrow, individual cells receive different cues from the body that control how they develop and mature. As a result, the HSCs differentiate into different types of blood cells that, once mature, circulate in peripheral blood. This process of differentiation, called hematopoiesis , is shown in more detail in [link] .

In terms of sheer numbers, the vast majority of HSCs become erythrocytes. Much smaller numbers become leukocytes and platelets. Leukocytes can be further subdivided into granulocytes , which are characterized by numerous granules visible in the cytoplasm, and agranulocytes , which lack granules. [link] provides an overview of the various types of formed elements, including their relative numbers, primary function, and lifespans.

A flowchart showing progression of development for formed elements of blood. At the top is a multipotent hematopoietic stem cell (hemocytoblast). This cell divides and after division some of the new cells remain stem cells. Others go down one of two paths depending on the chemical signals received. One path begins with lymphoid stem cells which can either become natural killer cells (large granular lymphocytes) or small lymphocytes. The natural killer cell is a medium-large purple cell. Small lymphocytes can either become T lymphocytes or B lymphoctyes. The T and B lymphocytes are medium size cells with a large nucleus. B lymphocytes become plasma cells which are medium size cells with a large nucleus. The other option for the stem cell is to become a myeloid stem cell. Myeloid stem cells follow one of four paths. One path leads to megakaryocyte which leads to platelets. Platelets are small flecks. The second path leads to erythrocyte. Erythrocytes are small donut shaped red cells. The third path leads to mast cells. The fourth path leads to basophil, neutrophil, eosinophil, or monocyte. Basophils are medium cells with many dark purple spots. Neutrophils are medium pink cells with a multi-lobbed nucleus. Eosinophils are medium size cells with many pink spots. Monocytes lead to macrophages or dendritic cells. Macrophages are large irregularly shaped cells. Dendritic cells have longer tendons branching off of them.
All the formed elements of the blood arise by differentiation of hematopoietic stem cells in the bone marrow.
A table of the formed elements. Top row reads: formed element, major subtypes, numbers present per microliter and mean, appearance in standard blood smear, summary of functions, and comments. The first row is for erythrocytes (red blood cells). There are 5.2 million per microliter of blood (ranging from 4.4 – 6 million). These cells are flattened biconcave disks with no nucleus and a pale red color. Their function is to transport oxygen and some carbon dioxide between tissue and lungs. Their lifespan is approximately 120 days. The set of rows is classified under leukocytes (white blood cells). Leukocytes as a group number 7000 per microliter of blood (ranging from 5000-10,000). Leukocytes have an obvious dark-staining nucleus and function in body defenses. They exit capillaries and move into tissues. Their lifespan is usually a few hours or days. Leukocytes are divided into two groups. The first is granulocytes including neutrophils, eosinophils, and basophils. The second is agranulocytes including lymphocytes and monocytes. Granulocytes number 4300 per microliter of blood (range of 1800-9950). Granulocytes have abundant granules in the cytoplasm and the nucleus is normally lobed. Granulocytes function in nonspecific (innate) resistance to disease and are classified according to membrane-bound granules in the cytoplasm. Neutrophils make up 50-70% of the total leukocytes and number 4150 per microliter of blood (range 1800-7300). Neutrophils have a nucleus with lobes that increase with age and pale lilac granules. They are phagocytic and particularly effective against bacteria; they release toxic chemicals from granules. Neutrophils are the most common leukocyte with a lifespan of minutes to days. Eosinophils make up 1-3% of total leukocytes. They number 165 per microliter of blood (range of 0 – 700). Eosinophils have a nucleus that is generally two-lobed and bright red-orange granules. They are phagocytic cells and particularly effective with antigen-antibody complexes. Eosinophils release antihistamines and increase allergies, they also help fight parasitic infections. Eosinophils have a lifespan of minutes to days. Basophils make up less than 1% of total leukocytes. They number 44 per microliter of blood (range 0 – 150). Basophils have a nucleus that is generally two lobed but difficult to see due to the presence of heavy, dense, dark purple granules. Basophils promote inflammation and are the least common leukocyte. Their lifespan is unknown. Agranulocytes (including lymphocytes and monocytes) number 2640 per microliter of blood (range 1700 – 4900). Agranulocytes lack abundant granules in the cytoplasm and have a simple-shaped nucleus that may be indented. They function in body defenses and are grouped into two major cell types from different lineages. Lymphocytes make up 20-40% of total leukocytes and number 2185 per microliter of blood (range 1500-4000). Lymphocytes are spherical cells with a single, often large, nucleus occupying much of the cell’s volume. They stain purple and are seen in large (natural killer cells) and small (B and T cells) variants. Lymphocytes are primarily involved in specific (adaptive) immunity. T cells directly attack other cells (cellular immunity); natural killer cells are similar to T cells but nonspecific. Lymphocytes originate in bone marrow but secondary production occurs in lymphatic tissue. Several distinct subtypes. Memory cells form after exposure to a pathogen and rapidly increase responses to subsequent exposure. Lifespan of many years. Monocytes make up 1-6% of total leukocytes. They number 455 per microliter of blood (range 200-950). Monocytes are large leukocytes with an indented or horseshoe-shaped nucleus. They are very effective phagocytic cells engulfing pathogens or worn out cells and also serve as antigen presenting cells (APCs) or other components of the immune system. Monocytes are produced in red bone marrow and are referred to as macrophages and dendritic cells after leaving circulation. The last row of the table is for platelets. These number 350,000 per microliter of blood (range 150,000-500,000). Platelets are cellular fragments surrounded by a plasma membrane and containing granules. They stain purple. The function of platelets is hemostasis plus releasing growth factors for repair and healing of tissues. They are formed from megakaryocytes that remain in the red bone marrow and shed platelets into circulation.
Formed elements of blood include erythrocytes (red blood cells), leukocytes (white blood cells), and platelets.

Granulocytes

The various types of granulocytes can be distinguished from one another in a blood smear by the appearance of their nuclei and the contents of their granules, which confer different traits, functions, and staining properties. The neutrophils , also called polymorphonuclear neutrophils (PMNs) , have a nucleus with three to five lobes and small, numerous, lilac-colored granules. Each lobe of the nucleus is connected by a thin strand of material to the other lobes. The eosinophils have fewer lobes in the nucleus (typically 2–3) and larger granules that stain reddish-orange. The basophils have a two-lobed nucleus and large granules that stain dark blue or purple ( [link] ).

Neutrophils have a multi-lobed nucleus. Eosinophils have a two-lobed nucleus and distinct pink spots when stained. Basophils have a two-lobed nucleus and distinct purple spots when stained. Each type of granulocyte is illustrated with a micrograph above it.
Granulocytes can be distinguished by the number of lobes in their nuclei and the staining properties of their granules. (credit “neutrophil” micrograph: modification of work by Ed Uthman)

Questions & Answers

pls someone should help with ANTIMICROBIAL MNEMONICS
Shuaib Reply
Aminoglycosides: Charasteristics of Aminoglycosides Nymonics; (AMINO)  Active against Aerobic gram negative Organisms Mechanism Of resistance is by Modifying enzymes Inhibit protein synthesis by binding to 30s ribosomal subunit Nephrotoxic Ototoxic Members  Of Aminoglycosides: Nymonics: (ST
Najeebul
Please can some briefly describe the germ theory?
Michael Reply
hyy all
Shuaib
pls someone should help with ANTIMICROBIAL MNEMONICS
Shuaib
which drug can be used to treat gonococus?
Lambon Reply
Madication - Antibiotic " and Gonorrhoea can be treat with Antibiotic
Maryam
Penicillin -An antibiotic
Michael
Ciprofloxacin tablets also an antibiotic
Michael
Ofloxacin is also antibiotics
Ikenna
yes
Elli
sory no fot gonococus
Elli
what are eukaryotes
Kakande Reply
eukaryotes simply refers to a cell who does not have a definite nucleus.
Franca
Eukaryotic cell has a true nucleus
arfa
what is right answer ?
krishna
I can remember now truly that eukaryotes have true nucleus.I was referring to procaryotes.
Franca
Eukaryotes have got true nuclues
Ede
they have a nucleus, larger than prokaryotes
Alanna
The eukaryotes cell are found in advance organisms.
Franca
Eukaryotes are multicellular organism which has a membrane bounded structure mostly found in animals and some plant cells
KINGSFORD
bacteria growth curve
sani Reply
No bacteria's don't grow curvey. Curve is a shape for a bacteria not growth
KINGSFORD
What is the difference between eukaryotes and prokaryotes?
KINGSFORD Reply
What's a multicellular organism?
KINGSFORD
multicellular organisms is an organism composed of many cells which are to varying degree integrated and independent.
Ruth
Boiling of milk to kill germs is called?
Kasigwa
pasteurization
Aiyedun
pasteurization
Katta
kadha
Katta
pasteurization
Gomathi
what is microscopic
monish
Pasteurization
Hosy
what are Flora
Hosy
you mean like the normal flora?
Tenucharity
pasteurization has to do with killing of the harmful microbes present for safe consumption. Eg. pasteurized yoghurt beside the bacteria that are supposed to be present, there may be harmful ones that have been killed
Tenucharity
Can someone explain flora
KINGSFORD
what is cell?
Anita Reply
the cell is the functional and structural unit of the body
Ruth
yes ur right
Kasigwa
who add me
Wali
hy
Katta
hy
Shivangi
basic structural, functional, and biological unit of all known living organisms. A cell is the smallest unit of life.
Haru
hi
Ruth
hi
Haru
coming Oct 2 I have a exam on microbiology
Haru
okay all the best
Ruth
hi
sachin
hi
Ruth
plz bro send question of microbiology
umesh
structural and functional unit of the life
monish
this is not the correct answer
Alanna Reply
why
Hayat
why not
Zaajid
what is the correct answer?
vannessa
where is question
Zaajid
ask you question
Zaajid
What are mitochondria
Hayat
what is mitochondria
Hayat
who is the father of microbiology
T-
not
Hayat
Antonie Van Luweenhoek
Ravikumar
Mitochondria is the powerhouse of the Cell. And is the cell organelle which helps cell in generation of energy by Cellular Respiration
Ravikumar
Mitochondria are threadlike structuresbounded by double membrane,and serves as the power house of the cell
Felicity
can I get notes on introduction to microbiology just started I have no idea please
rita
Mitochondria is a membrane bounded structure, it's sausage in shape and has inner and outer membrane. The inner membrane is folded at the surface called cristae which project to form matrix. They function as the power house for the cell.
KINGSFORD
Rita are you a medilab student?
KINGSFORD
the father of microbiology is Luis pasture
Able
Erythromycin--------(+++) Cefuroxine ------------(++++) Chloramphenicol --(++) Vancomycin ---------(++++) please what does the above result means. under urine culture and sensitivity?
Kwame
francesco redi was the first to challenge spontaneous generation not pasteur
Alanna
Rita are you a medilab student? no please am a public health student
rita
kwame Evans this result ll mean that the bacterial in the urine can be remedy / killed by this antibacterial (cefuroxime and the rest are antibiotics
rita
Rita Appiah thanks.
Kwame
contact backstage
Emmanuel
what is a vacual
Star Reply
What is microbiology
Nji Reply
it is the study of microscopic organisms and basically their relation to the environment
Tenucharity
what is elements
Erneet Reply
an Element is a substance that can not be further decomposed by ordinary chemical means.
vannessa
can i get notes of biochemistry
Sneha Reply
yep why not
Abdullah
what is microbiology?
Gayanjali Reply
what is malaria
Gayanjali
bios- life/living things micro- small things that cannot be seen with the naked eye In other words the study of living things that cannot be seen with the naked eye but with the help of a microscope
Coltuneac
logos- science
Coltuneac
it is 10 to the power minus 6 biology which studies life forms on those scales including metabolism , physiology, nutrition requirements
barun
what are prokarytes
NDIMUKIKA Reply
A prokaryotic is a unicellular organanism that lacks a membrane bound nucleus ,mitochondria or other any membrane bound organelle. .......
Maryam
in prokaryotic nucleus is not developed and cell organelles are absent
Harshad
how is one who is feeling blockage in the chest with mucus be diagnosed
Essy Reply
be talking sepitol every morning
Ikenna
is that all?
Ven.phumie

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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