In this article we will discuss about:- 1. Meaning and Classification of Leukocytes 2. Functions of Leukocytes 3. Diseases.
Meaning and Classification of Leukocytes:
WBC or leukocytes are body’s protective system. They have the ability to “seek out and destroy” the foreign invader. Thus, they protect us from bacteria, virus, fungus and parasite
1. Granulocytes (contain granules) ― Polymorphonuclear:
(i) Neutrophils
(ii) Eosinophils
(iii) Basophils
2. Agranulocytes (do not contain granules):
(i) Lymphocytes
(ii) Monocytes
Concentration of Various WBC in Blood:
i. Polymorphonuclear neutrophils – 55-60%
ii. Polymorphonuclear eosinophils – 2-3%
iii. Polymorphonuclear basophils – 0-1%
iv. Monocytes – 4-5%
v. Lymphocytes – 30-35%
Functions of Leukocytes:
i. Granulocytes and monocytes protect the body against the invading organism by phagocytosis.
ii. Lymphocytes are responsible for immunity.
Staining of WBC:
Peripheral smear is stained by Leishman’s stain a variant of Romanowsky’s stain. It helps to identify the different types of WBC.
Other variants of Romanowsky’s stain are:
i. Giemsa’s
ii. Jenner
iii. Wrights
Concept of Pools:
There are three different pools, where the different types of WBC are seen.
i. Marrow Pool:
90% of neutrophils are in marrow pool, i.e. red bone marrow.
ii. Blood Pool:
3% of neutrophils are in blood pool.
It is further divided into:
a. Circulating pool (in flowing blood)
b. Marginal pool (adhering to vascular margin)
c. Development of granulocyte through various series is called myeloid series.
d. Process of granulopoieses takes 12 days.
e. More than 3 times granulocytes are stored in bone marrow.
Tissue Pool:
7% of neutrophils are in tissue pool.
Formation of Lymphoid Series:
Lymphoblast → Prolymphocyte → Lymphocyte
Regulation of Leucopoiesis:
i. Colony Stimulating Factor:
Cytokines that stimulate the formation of different types of granulocytes are called colony stimulating factor.
They are:
a. G-CSF (granulocyte CSF)
b. M-CSF (monocyte CSF)
c. GM-CSF (granulocyte monocyte CSF)
ii. Interleukins:
Interleukins are cytokines that stimulate lymphocyte formation.
Neutrophils:
Diameter – 10-14 µm
Cytoplasm – Fine pinpoint granules. These granules contain glycosidase, sulphatase, phosphatase, nuclease and peroxidase.
Nucleus:
i. Young Neutrophils: Single horseshoe shaped nucleus
ii. Mature Neutrophils: 2 to 6 lobes
iii. Nuclei are connected by chromatin filament.
Functions:
i. 1st line of defense
ii. Neutrophils function by the process of phagocytosis.
Phogocytosis:
i. Phagocytosis is also called as “cell eating”.
Various steps in phagocytosis are:
1. Margination:
The process of attachment of neutrophils to the capillary endothelium is called margination. Cell adhesion molecules are present on the endothelial surface that helps the neutrophils in margination.
2. Diapedesis:
Marginated neutrophils squeeze through intercellular spaces in blood vessel and pass out.
3. Chemotaxis (Chemo-Chemical, Taxis-Movement):
Movement of neutrophils towards the chemical substance present at the site of inflammation is called chemotaxis.
Substances that mediate chemotaxis are called chemokines. Some of the chemokines are C5, leukotriene B4, and cytokines.
4. Opsonization:
Process of coating of bacteria by opsonin, by which bacteria becomes tasty for phagocytosis is called opsonization, e.g. IgG opsonin.
Engulfment:
i. Pseudopodia are put out in all direction.
ii. Pseudopodia meet each other and fuse to form phagocytic vesicle.
iii. Single neutrophil can phagocytize 3-20 bacteria before it dies.
Degranulation:
Proteolytic enzymes present in neutrophils digest the bacteria.
In macrophages, lipases and digest the bacteria.
Phagocytosis by Macrophage:
i. Monocytes in blood when they enter into tissue, become macrophages.
ii. They phagocytize around 100 bacteria.
iii. Lipases digest the bacterial wall.
iv. Oxidizing agents that help in killing the bacteria are:
a. Superoxide
b. Hydrogen peroxide
c. Hydroxyl ions
v. Enzyme myeloperoxidase is also a powerful bacterial agent.
Applied Physiology:
Tubercle bacilli are resistant to killing effects of neutrophils and macrophages leading to chronic disease.
Phagoctytosis by Neutrophils:
(i) 3-20 bacteria are killed
(ii) Neutrophils are killed during phagocytosis
(iii) Phagocytize smaller particles
(iv) Particles: Not more than the size of bacteria
Phagocytosis by Macrophages:
(i) About 100 bacteria are killed
(ii) Macrophages are alive even after phagocytosis
(iii) Phagocytize larger particles
(iv) Bacteria and malarial parasites
Variation in Neutrophil Count:
Neutrophilia (>15,0U0/mm3):
Increase in neutrophil count is called as neutrophilia.
Physiological:
Newborn, exercise and emotion.
Pathological:
i. Pyogenic bacterial infections
ii. Acute rheumatic fever
iii. Burns
iv. Myocardial infarction.
Neutropenia (< 2500/mm3):
i. Typhoid
ii. Aplasia of bone marrow
iii. Bone marrow depression due to:
a. Drugs, e.g. Chloromycetin
b. Radiations, e.g. X-rays.
Arneth Count:
Counting the number of lobes in neutrophils and expressing it as % of cells with different number of lobes is called Arneth count or Cooke’s Arneth count. Shift to left ― (N1 + N2 + N3).
Indicates hyperactive bone marrow.
Shift to right ― (N4 + N5)
Indicates hypoactive bone marrow.
Eosinophils:
Diameter – 10-14 µm
Cytoplasm – Acidophilic coarse granules
Granules contain major basic protein, eosinophilic cationic protein, lyophospholipase, aryl sulphatase and histaminase.
Nucleus:
Bilobed connected by chromatin strand.
Functions:
Eosinophils are weak phagocytes. They act as parasiticide and larvicide by releasing hydrolytic enzymes and highly reactive forms of oxygen.
Role of Eosinophils in Allergy (Lungs and Skin):
Basophils and mast cells release → Eosinophils chemotactic factor → Eosinophils migrate towards allergic tissue → Detoxify inflammation inducing substances → Destroy the allergen-antibody complexes → Prevents spread of local inflammatory process.
Eosinophilia:
Allergy, asthma, hay fever, skin diseases, e.g. urticaria, eczema, parasitic infection, e.g. round worm, hookworm, and tapeworm.
Eosinopenia:
i. ACTH and steroid therapy
ii. Stress
Lymphocytes:
Lymphocytes are agranular cells. There are small and large lymphocytes.
Small Lymphocytes:
Size – 8 µm in diameter
Cytoplasm – Very thin rim of cytoplasm is seen
Nucleus – Large nucleus staining deeply with basic dyes
Function – Responsible for immunity of a person
Large Lymphocyte:
They are younger form of lymphocytes
Size – 10 µm in diameter
Cytoplasm – Clear blue cytoplasm
Nucleus – Oval or kidney shaped nucleus.
Basophils:
Size – 10-14 µm
Cytoplasm – Basophilic cytoplasm is full of granules- coarse in nature and blue black in color. Granules contain: Heparin, histamine, 5-hydroxytryptamine (serotonin).
Nucleus – It has 2 lobes connected by a strand. It is spectacle shaped.
Function – Mild phagocytosis plays a role in allergic reaction release heparin that prevents clotting.
Basophilia:
i. Viral infections
ii. Allergic reactions
iii. Chronic myeloid leukemia
Basopenia:
i. Steroid therapy
ii. Drug induced reaction.
Monocytes:
Size – 18-25 µm, largest of the leukocytes
Cytoplasm – Clear without granules
Nucleus – Horseshoe or kidney shaped and is situated eccentrically.
Functions:
1. Role in defense by phagocytosis.
2. Role in tumor immunity: From blood these cells enter into tissues to form tissue macrophages (60 to 80 µm in size).
3. Monocytes produce Cytokines, Hemopoietic factors TNF
They circulate in blood for 72 hours and leave the blood and enter into tissues to blood for 72 hours and leave the blood and enter into tissues to become tissue macrophages. In tissues they live for months.
Monocytosis:
i. Increase in monocyte
ii. Certain bacterial infections, e.g. tuberculosis
iii. Infectious mononucleosis
iv. Monocytic leukemia.
Monocytopenia:
i. Decrease in monocytes
ii. Occurs in hypo-plastic marrow.
Mast Cells:
i. Located outside the capillaries in the body.
ii. Mast cells liberate heparin into blood. This prevents blood coagulation.
Mast Cells in allergic Reaction:
Antigen + antibody IgE Mast cells rupture Release of histamine, serotonin, bradykinin, SRS―of anaphylaxis, lysosomal enzymes, heparin cause Local vascular and tissue reactions Responsible for allergic manifestations
Inflammation:
When injury occurs to the tissues by ― Bacteria, trauma, chemical, heat or some other way, multiple substances are released to produce local reaction called inflammation.
Inflammation is characterized by:
i. Vasodilatation of local blood vessels (Redness or Rubor).
ii. Increased permeability of capillaries and leakage of fluid into interstitial space.
iii. Clotting of fluid due to leakage of fibrinogen from capillaries.
iv. Migration of granulocytes and monocytes into tissues.
v. Swelling of tissue cells (tumor).
Inflammatory changes—clinically are:
i. Rubor ― Redness
ii. Tumor ― Swelling
iii. Calor ― Heat
iv. Dolor ― Pain
v. Loss of function
Physiologically, steps of inflammation are:
i. Vasodilation
ii. Leukocyte emigration
iii. Chemotaxis
iv. Phagocytosis
Inflammation causes “Walling off” effect.
This effect delays the spread of infection.
Streptococcal Infection:
(i) Walling off occurs slowly
(ii) Grater tendency to spread.
Staphylococcal Infection:
(i) Walled off rapidly
(ii) Prevented from spreading to other parts.
Various Lines of Defense:
1. Tissue Macrophage ― First Line of Defense:
When an organism enters the body, the tissue macrophages enlarge in size and move towards the site of infection forming the 1st line of defense. For example alveolar macrophages in lungs, microglia in brain, and histiocytes in skin.
2. Neutrophils ― Second Line of Defense:
Within 1 hour after the entry of the organism, neutrophils invade the inflamed area.
The steps involved are:
i. Margination
ii. Diapedesis
iii. Chemotaxis
iv. Phagocytosis.
3. Second Macrophage Invasion: 3rd Line of Defense:
Along with neutrophils, monocytes enter into circulation. Monocytes need 8 hours to swell and acquire tremendous lysosomes and become macrophages. Now, macrophages can phagocytize more bacteria.
4. Increased Production of Granulocyte and Monocyte by Bone Marrow ― 4th Line of Defense:
Bone marrow starts producing more number of granulocyte and monocytes. It takes 3-4 days for the bone marrow to form new cells and release them into blood.
Diseases of Leukocytes:
I. Leukemia:
Leukemia is a malignant disease of white blood cells. There is uncontrolled production of white blood cells with plenty of immature cells.
Classification:
1. Depending on cell type, leukemia can be classified as:
i. Lymphocytic leukemia
ii. Myeloid leukemia.
2. Depending on the time duration, leukemia can be classified as:
i. Acute leukemia
ii. Chronic leukemia.
In lymphocytic leukemia, there is uncontrolled production of lymphoid cells.
In myeloid leukemia, granulocyte and its precursors are increased.
Myeloid leukemia can be:
i. Neutrophilic leukemia
ii. Eosinophilic leukemia
iii. Basophilic leukemia
iv. Monocytic leukemia.
Leukemia can be:
i. Well differentiated
ii. Undifferentiated
In well differentiated type, the disease will have a chronic course over 10-20 year.
In undifferentiated type, the disease will have acute course and often leads to death within few months if untreated.
Causes of Leukemia:
i. Ionizing radiation, cytotoxic drugs, mutations and immune deficiency states.
ii. Extra medullary hematopoiesis takes place leading to enlargement of liver, spleen and lymph nodes. Leukemic cells of bone marrow invade the surrounding bone leading to bone pain and a tendency to fracture easily.
iii. Bleeding tendency such as purpura, epistaxis and gum bleed occurs. Diagnosis is not obvious and is uncovered by laboratory investigations.
Peripheral smear shows blast cells.
iv. Anemia and thrombocytopenia occurs (leading to bleeding).
II. Leukopenia:
Decrease in the number of WBC count leaves the body unprotected against disease.
Causes are irradiation, X-ray or gamma rays and exposure to drugs and chemicals.
Treatment with antibiotics and blood transfusion will improve the condition.