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Leukocytes – (White Blood Cells)

Leukocytes can be classified as accordingly:

Cell Type          Abundance (%) Diameter (µm)

Neutrophils      60-70                    12-15

Eosinophils       2-4                         12-15

Basophils          0-1                         12-15

Lymphocytes    20-30                      6-18

Monocytes       3-8                          12-20

(Cells in red are agranular, they are agranulocytes. Cells in blue are granular, they are granulocytes)

Leukocytes leave blood capillaries by passing between endothelial cells and penetrating into the connective tissue by a process known as diapedesis. Diapedesis is increased in individuals infected with micro-organisms.

Neutrophils (Polymorphonuclear Leukocytes)

Neutrophils constituting around 60-70% of the total circulating leukocytes and they have a diameter of 12-15µm. Neutrophils are polynuclear in appearance however these lobes are in fact joined. (May have 2-5 lobes however, 3 seems average). Neutrophils are also granular.

Neutrophils are adept at surviving in aerobic conditions, making them useful for killing bacteria and cleaning up debris in low oxygenated environments e.g. inflamed or necrotic tissue.

Neutrophils are relatively short lived with a life span of only 1-4 days, where-after they die by apoptosis.

Neutrophils act as a defence against micro-organism invasion, especially bacteria. They are active phagocytes of small particles and are sometimes known as microphages to distinguish them from macrophages which are larger cells.

Bacteria adhere to neutrophil surfaces, where they are phagocytosed. They then occupy vacuoles within the cell (phagosomes). The granules fuse with the phagosomes releasing their content – enzymes which kill and digest the micro-organisms


Eosinophils are much less numerous than neutrophils (2-4%). They are characterised by their bilobed nucleus and the high affinity their granules have for eosin.

The role of eosinophils in the immune system is to fight against parasitic organisms (Eosinophils are of much higher concentrations during parasite attacks and allergic reactions). They secrete digestive enzymes directly onto the parasite which may also result in localised tissue damage.


Basophils make up less than 1% of the total blood leukocyte count; they are therefore rare to find in blood smears. Their nuclei are divided into irregular lobes which are hard to distinguish because of the overlying granules.

Basophils contain specific granules which themselves contain heparin and histamine, they are capable of producing leukotrienes (fatty molecules which are used as paracrine/autocrine signalling molecules), these leukotrienes can cause slow contractions of smooth muscle.

It is believed that basophils supplement mast cell function in immediate hypersensitivity reactions by migrating into the connective tissues.


Lymphocytes can be classified into two primary groups (depending on their specific surface receptor), either T or B lymphocytes. Both T and B lymphocytes possess specific antigen receptors on their surface, which is important in immunological responses (see later articles for immunology and lymphocytes)

Lymphocytes vary greatly in lifespan, from a few days to years.

Lymphocytes are also the only leukocytes which can return from tissues back to the blood after diapedesis.


The nuclei of monocytes are generally oval, horseshoe or kidney shaped. Because of the delicate chromatin distribution in monocytes they stain lighter than the larger lymphocytes – aiding identification

After crossing capillary walls and entering connective tissues, monocytes differentiate into macrophages


Platelets (thrombocytes) are non-nucleated cell fragments 2-4µm in diameter (the small specs in the picture.) They originate from fragmentation of megakaryocytes residing in the bone marrow. The primary role of platelets is the promotion of blood clotting and the repair of gaps in the walls of blood vessels.

The Role of Platelets in the Clotting Cascade

1.Primary Aggregation: Damage to the endothelium of blood vessels causes absorption of plasma proteins on the subjacent collagen. Platelets immediately begin to aggregate on this damaged tissue forming a platelet plug.

2.Secondary Aggregation: Platelets in the plug release the contents of their granules, ADP s a potent inducer of platelet aggregation.

3.Blood Coagulation: Factors from platelets, damaged blood vessels and blood plasma promote a cascade of approximately 13 proteins which give rise to the formation of fibrin. Fibrin forms a 3D network of fibres which trap red blood cells leukocytes and platelets to form a blood clot.

4.Clot Retraction: This is when the formed clot which bulges into the blood vessel begins to contract. This is due to the interaction of platelet actin, myosin and ATP.

5.Clot Removal: The blood vessel wall is repaired by new tissue formation. The clot is then removed by the enzyme plasmin.

Recommended further reading on this subject, specifically the platelet plug formation and the clotting cascade can be found at:

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