Anatomy and Physiology of White Blood Cells (Leukocytes)

What are the main characteristics of leukocytes?

• 5 types of WBC

• Have a nucleus and are colorless in fresh blood

• They comprise the cellular components of the immune system

• They migrate through capillary walls into body tissues

What stains do we use when looking at blood stains?

• Romanovsky Type Stains

  • Mixture of basic dye (methylene blue) and an acidic dye (eosin)

  • Used specifically for staining blood smears

What are the basis of creating subdivisions of WBCs?

• Nuclear appearance

• Specific staining affinity of cytoplasmic granules with romanovsky type stains

  • Is there regular or irregular cytoplasm, is there a lot, a little, are there granules?

What categories are the WBCs divided into?

1.) Granulocytes

(polymorphs/polymorphonculear) - irregular nucleus

• Neutrophils

• Eosinophils

• Basophiles

2.) Agranulocytes

(Mononuclear) - round/slightly indented nuclei

Lack granulocytes

• Lymphocytes

• Monocytes

What type of WBC is this?

Neutrophil

What do we need to know about the proportions of WBC in an animal?

WBC look different in different species, but proportions in a blood sample also vary depending on the species you are looking at.

• % of each type of WBC varies depending on species

• In all species, Basophils are present in the smallest number

• The largest numbers are typically monocytes and lymphocytes

What is hematopoeisis?

process describing the formation of the formed elements (everything other than plasma) of blood occur in the blood marrow

• All begins with multipotent stem cells

• Early subdivision into two two lineages (Myeloid and Lymphoid)

What is granulopoeisis?

• Process giving rise to granulocytic series of WBCs in the bone marrow

• We should not see immature forms of the WBCs in the blood, if we see band neutrophils, it may indicate infection

What are the main features of neutrophils?

Also called Polymorphs

• Are granulocytes

• Approximately 12um diameter

  • Quite large, an RBC - usually 5um

• Characteristic segmented nucleus with 3-5 lobes, more as the cell matures

• 2 types of granules (specific and non-specific)

  • Specific staining granules too small to be seen in the dog

  • Some species variation

• Commonest type of WBC in dog blood (But not true in all species)

What is the lifespan like in a neutrophil?

What is their distribution like in the body/circulation?

What are their main functions?

If they cannot perform their main functions what happens?

• Maturation process in BM = 6 days, to be released with 3 lobes into circulation

  • Bone marrow - circulating neutrophil pool

  • Marginating neutrophil pool - passively delay transit time (spleen and lung)

• Only the cells in the circulating pool are picked up during blood sampling (As opposed to the marginating neutrophil pool)

  • If they are not required, they will pass out of the vascular system, enter tissues to perform function OR

  • In 7-14 hours they will be lost from body surfaces or destroyed by macrophages

• Functionally:

  • First responders to infection, the innate inflammatory response

  • Occurs in the connective tissue space

  • No re-entry into the circulation after migration into tissue, they can last here for 1-4 days

How do white blood cells exit the vascular system?

• Extravasation, leukocyte adhesion cascade or diapedesis

  • Neutrophils from vascular system enters the connective tissue by squeezing between endothelial cells lining the blood vessels (post capillary venules)

• 4 STEPS:

1.) Chemoattraction

2.) Rolling adhesion

3.) Tight adhesion

4.) Transmigration

**ALL WBC enter connective tissue via this route

What are the main functions of neutrophils?

• First responders to infection/part of the innate inflammatory response

• Cell is an active phagocyte (Goes around like “pacman” and engulfs bacteria, granules help break down substances)

  • Cell can only do this so many times before dying (puss)

• Can modulate inflammatory response (bring in other cells to respond), and tissue damage

What contributes to the formation of pus and fever?

Dead neutrophils form puss

When they burst open, they release pyrogens that cause fever

What is neutrophilia?

• Ongoing infection, characterized by release of immature forms (Band neutrophils) from the bone marrow

What are the main features of eosinophils?

• Granulocyte

• 2-8% of total WBC

• Typically a bi-lobed nucleus

• Characteristic features are - eosinophilic “pink” staining cytoplasmic granules

• Species differences

What type of WBC is this?

Eosinophil

What are the lifespan of eosinophils?

What is their main function?

• Take 2-6 days to develop in BM, spending 1-24 hrs in circulation

  • Exit to the tissue (skin, GI, lungs) that is often exposed to the environment, lasts for 2 days to 2 weeks

• Functions:

  • Active participants in many immune responses (e.g. phagocytosis of Ag/Ab complexes)

  • Regulate Inflammation

    • Histamine and enhance the release of histamine and serotonin from other cells of the immune system

  • Beneficial role in host defense against nematode and other parasitic infections

    • e.g. they degranulate ont the surface of helminths

**But can also be damaging as part of the inflammatory process of allergic diseases, e.g. eosinophils and asthma

What are the main features of basophils?

• Granulocyte

• Rare (0-0.5% of total WBC)

• Bi-lobed or multi-lobed nucleus

• Main characteristics: purple (Basophilic-blue), specific staining cytoplasmic granules

• Species variation in number and staining

What WBC is this?

Basophil

What are the main functions of basophils?

• Role in inflammatory and allergic type reactions

• Non-specific and limited phagocytic activity

• De-granulate on contact with antigens, separate foreign pathogens out so immune system can better deal with them

• Granules contain…

  • Vasodilantion

  • Anticoagulant

• Supplement the action of mast cells (Sit in the connective tissues, when stimulated burst open like basophils, similar roles but are DIFFERENT)

  **Play a role in severe allergic reactions - release of leukotriene, which promotes inflammation, causing airway constriction

• Lifespan, thought to be a few days

What are the main functions of monocytes?

• Agranulocyte

• Largest type of WBC: 12-18um

• 3-8% of total WBC

• Variably shaped nucleus

• Cytoplasm abundant greyish blue, often foamy or vacuolated

  • LOTS of cytoplasm, not clear

• Develop and stored in bone marrow and spleen

What type of WBC is this?

Monocyte

What is the lifespan and function of a monocyte?

• Have a short, transient time in bood, are recruited into the tissue (Connective) where they differentiate into connective tissue macrophages

• Variable lifespan

  • Macrophages are present in the immune system within connective tissue, ready to deal with threats

What are the features of lymphocytes?

• Round, densely stained nucleus and scant but intensely stained “skyblue” cytoplasm

• Approximate size of RBC

  • Note that medium and large lymphocytes are found in some species

What are the functions of lymphocytes?

• Humoral immunity (B cells) + Cellular Immunity (T cells)

• Different populations in blood

  • T-Cells: mediated immune reactions, leave bone marrow and mature in thymus

  • B cells: antibody production, lymphopoeisis completed in bone marrow

• Circulating B and T lymphocytes cannot be easily distinguished in blood smears

What type of WBC is this?

Lymphocyte

What is the migration system of lymphocytes like?

• Migrate and are stored in secondary lymphatic organs and tissues

• Have a variable lifespan

What animal blood is this?

Canine

What animal blood is this?

Feline

What animal blood is this?

Equine

What animal blood is this?

Bovine