Sangue

μL.

They are also larger than erythrocytes and are the only formed elements that are

complete cells, possessing a nucleus and organelles.

There are many types of leukocytes. Most of these types have a much shorter lifespan

than that of erythrocytes, some as short as a few hours or even a few minutes in the

case of acute infection.

One of the most distinctive characteristics of leukocytes is their movement. Whereas

erythrocytes spend their days circulating within the blood vessels, leukocytes routinely

leave the bloodstream to perform their defensive functions in the body’s tissues. For

leukocytes, the vascular network is simply a highway they travel and soon exit to

reach their true Destination → emigration or diapedesis in which they squeeze through

adjacent cells in a blood vessel wall.

Leukocytes could be divided into two groups, according to whether their cytoplasm

contained highly visible granules:

Granular leukocytes contain abundant granules within the cytoplasm. They include

neutrophils, eosinophils, and basophils

Agranular leukocytes, in which granules are not totally lacking , they are far fewer and

less obvious. Agranular leukocytes include monocytes, which mature into

macrophages that are phagocytic, and lymphocytes, which arise from the lymphoid

stem cell line

All of these are produced in the red bone marrow and have a short lifespan of hours to

days. They typically have a lobed nucleus and are classified according to which type of

stain best highlights their granules.

1. Neutrophils:

most common, 50–70 % of total leukocyte count 10–12 μm in diameter

(significantly larger than erythrocytes) They are called neutrophils because their

granules show up most clearly with stains that are chemically neutral (neither

acidic nor basic).

The granules are numerous but quite fine and normally appear light lilac

The nucleus has a distinct lobed appearance and may have two to five lobes

(the number increasing with the age of the cell). Neutrophils are rapid

responders to the site of infection and are efficient phagocytes with a

preference for bacteria. Their granules include:

lysozyme, an enzyme capable of lysing, or breaking down, bacterial cell

 walls;

oxidants such as hydrogen peroxide;

 defensins, proteins that bind to and puncture bacterial and fungal

 plasma membranes, so that the cell contents leak out.

Granular leukocytes

The leukocytes that contain cytoplasmic granules and lobulated nuclei are the

polymorphonuclear granulocytes, of which the neutrophils (1) are the most abundant.

The neutrophil cytoplasm contains fine violet or pink granules that are difficult to see

with a light microscope. As a result, the cytoplasm appears clear or neutral. The

nucleus consists of several lobes connected by narrow chromatin strands. Immature

neutrophils contain fewer nuclear lobes.

2. Eosinophils:

2–4 % of total leukocyte count.

10–12 μm in diameter.

The granules of eosinophils stain best with an acidic stain known as eosin. The nucleus

of the eosinophil will typically have 2-3 lobes and, if stained properly, the granules will

have a distinct red to orange color. The granules of eosinophils include:

• antihistamine molecules, which counteract the activities of histamines,

inflammatory chemicals produced by basophils and mast cells.

• molecules toxic to parasitic worms, Eosinophils are also capable of

phagocytosis. Implied in allergies and parasitic worm infestations, and

autoimmune diseases.

Eosinophils (1) are identified in a blood smear by their cytoplasm, which is filled with

distinct, large, eosinophilic (bright pink) granules. The nucleus in eosinophils typically

is bilobed, but a small third lobe may be present.

Granular leukocytes <1% of the total leukocyte count

Slightly smaller than neutrophils and eosinophils at 8–10 μm in diameter. The granules

of basophils stain best with basic (alkaline) stains. Basophils contain large granules

that pick up a dark blue stain and are so common they may make it difficult to see the

two-lobed nucleus. In general, basophils intensify the inflammatory response. The

granules of basophils release histamines, which contribute to inflammation, and

heparin, which opposes blood clotting.

The granules in basophils (1) are not as numerous as in eosinophils. However, they are

more variable in size, less densely packed, and stain dark blue or brown. Although the

nucleus is not lobulated and stains palely basophilic, it is usually obscured by the

density and number of granules.

Agranular leukocytes contain smaller, less-visible granules in their cytoplasm. Nucleus

is simple in shape, sometimes with an indentation but without distinct lobes. 1.

Lymphocytes are the only formed element of blood that arises from lymphoid stem

cells. 20–30 % of all leukocytes. The size range of lymphocytes is quite extensive. Two

size classes:

• the large cells are 10–14 μm and have a smaller nucleus-to-cytoplasm ratio and

more granules → The larger lymphocytes are typically NK cells

• the smaller cells are typically 6–9 μm with a larger volume of nucleus to cytoplasm,

creating a “halo” effect → Smaller lymphocytes are either B or T cells.

Natural killer (NK) cells are capable of recognizing cells that do not express “self”

proteins on their plasma membrane (e.g. cancer cells, cells infected with a virus) →

they provide generalized, nonspecific immunity. B lymphocytes and T lymphocytes

play prominent roles in defending the body against specific pathogens and are

involved in specific immunity. B cells produces the antibodies that bind to specific

foreign or abnormal components of plasma membranes. T cells provide cellular-level

immunity by physically attacking foreign or diseased cells.

A memory cell is a variety of both B and T cells that forms after exposure to a

pathogen and mounts rapid responses upon subsequent exposures. Unlike other

leukocytes, memory cells live for many years.

Agranular leukocytes have few or no cytoplasmic granules and exhibit round to nuclei.

Lymphocytes vary in size from cells smaller than erythrocytes to cells almost twice

as large. For a size comparison between lymphocytes and erythrocytes, this

illustration of a human blood smear depicts a large lymphocyte (1) and a small

lymphocyte (2) surrounded by the red-staining erythrocytes. In small lymphocytes

(2), the densely stained nucleus occupies most of the cytoplasm, which appears as a

thin basophilic rim around the nucleus. The cytoplasm in lymphocytes is usually

agranular but may sometimes contain a few granules. In large lymphocytes (1), the

basophilic cytoplasm is more abundant, and the larger and paler nucleus may contain

one or two nucleoli.

2. Monocytes originate from myeloid stem cells. 2–8 % of the total leukocyte

count. They are typically easily recognized by their large size of 12–20 μm and

indented or horseshoe-shaped nuclei. Macrophages are monocytes that have

left the circulation and phagocytize debris, foreign pathogens, worn-out

erythrocytes, and many other dead, worn out, or damaged cells. Macrophages

also release antimicrobial defensins and chemotactic chemicals that attract

other leukocytes to the site of an infection.

Monocytes (1) are the largest agranular leukocytes. The nucleus varies from round or

oval to indented or horseshoe shaped and stains lighter than the lymphocyte nucleus.

The abundant cytoplasm is lightly basophilic with few fine granules.

Platelets (PLTs)

Platelet is not a cell but rather a fragment of the cytoplasm of a cell called a

megakaryocyte that is surrounded by a plasma membrane. Megakaryocytes are

descended from myeloid stem cells and are large, typically 50–100 μm in diameter,

and contain an enlarged, lobed nucleus.

Megakaryocytes remain within bone marrow tissue and ultimately form platelet-

precursor extensions that extend through the walls of bone marrow capillaries to

release into the circulation thousands of cytoplasmic fragments, each enclosed by a

bit of plasma membrane. These enclosed fragments are platelets.

Each megakarocyte releases 2000–3000 platelets during its lifespan. Platelets are

relatively small, 2–4 μm in diameter, but numerous. After entering the circulation,

approximately one-third migrate to the spleen for storage for later release in response

to any rupture in a blood vessel. Platelets remain only about 10 days, then are

phagocytized by macrophages. Platelets are critical to hemostasis, the stoppage of

blood flow following damage to a vessel. They also secrete a variety of growth factors

essential for growth and repair of tissue, particularly connective tissue.

The blood

A high-magnification photomicrograph of a human blood smear shows erythrocytes

(3), a basophil (1), a neutrophil (5), and platelets (4). The basophil (1)

cytoplasm is filled with dense basophilic granules (2) that obscure the nucleus. In

contrast, the neutrophil (5) cytoplasm does not show granules, and its nucleus is

multilobed (6).

A high-magnification photomicrograph shows numerous erythrocytes (1), platelets (2),

and a large monocyte (3) with a characteristic kidney-shaped nucleus and nongranular

cytoplasm.

Microscopic examination of a stained blood smear reveals the major blood cell types.

Erythrocytes, or RBCs, are non-nucleated cells and are the most numerous blood cells.

In contrast, leukocytes, or WBCs, are nucleated and subdivided into granulocytes and

agranulocytes, depending on the presence or absence of granules in their cytoplasm.

Granulocytes are the neutrophils, eosinophils, and basophils. Agranulocytes are the

monocytes and lymphocytes. Platelets or thrombocytes are not blood cells. Instead,

they are the smallest, nonnucleated formed elements that appear in the blood of all

mammals. Platelets are membrane-bound cytoplasmic fragments or remnants of

megakaryocytes.

Blood typing

Blood transfusions in humans were risky procedures until the discovery of the major

human blood groups by Karl Landsteiner, an Austrian biologist and physician, in 1900.

Until that point, physicians did not understand that death sometimes followed blood

transfusions, when the type of donor blood infused into the patient was incompatible

with the patient’s own blood. Blood groups are determined by the presence or absence

of specific marker molecules on the plasma membranes of erythrocytes (antigenes).

With their discovery, it became possible for the first time to match patient-donor blood

types and prevent transfusion reactions and deaths.

Antigens are substances that the body does not recognize as belonging to the “self”

and that therefore trigger a defensive response from the leukocytes of the immune

system.

AB, RH antigen

AB blood antigens

ABO blood typing designates the presence or absence of two antigens, A and B.

People whose erythrocytes have A antigens on their erythrocyte membrane

 surfaces are designated blood typ