The total volume of blood in an average person is approximately 5 L; it is a specialized type of connective tissue, composed of cells, cell fragments,

and plasma, a fluid extracellular element. Blood circulates throughout the body and is well adapted for its manifold functions in transporting nutrients, oxygen, waste products, carbon dioxide, hormones, cells, and other substances. Moreover, blood also functions in the maintenance of body temperature.

FORMED ELEMENTS OF BLOOD

The formed elements of blood are red blood cells (erythrocytes), white blood cells (leukocytes), and platelets. The nomenclature developed for these formed elements is based on their colorations with Wright’s or Giemsa’s modification of the Romanovsky-type stains as applied to blood and marrow smears used in hematology. (Table 5-1)

•Red blood cells (RBCs), the most populous, are anucleated and function entirely within the circulatory system by transporting oxygen and carbon dioxide

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to and from the tissues of the body (see Chapter 12, Respiratory System).

•White blood cells (WBCs) perform their functions outside the circulatory system and use the bloodstream as a mode of transportation to reach their destinations.

•There are two major categories of white blood cells, agranulocytes and granulocytes. Lymphocytes and monocytes compose the first group, whereas neutrophils, eosinophils, and basophils compose the latter and are recognizable by their distinctive specific granules.

Lymphocytes are the basic cells of the immune system and, although there are three categories (T lymphocytes, B lymphocytes, and null cells), special immunocytochemical techniques are necessary for their identification.

When monocytes leave the bloodstream and enter the connective tissue spaces, they become known as macrophages, cells that function in phagocytosis of particulate matter, as well as in assisting lymphocytes in their immunologic activities (see Chapter 9, Lymphoid System).

ity to stains. Neutrophils function in phagocytosis of bacteria, and because of that, they are frequently referred to as microphages.

Eosinophils stain a reddish-orange color; they participate in antiparasitic activities and phagocytose antigen-antibody complexes.

Basophils stain a dark blue color with dyes used in studying blood preparations. Although the precise function of basophils is unknown, the contents of their granules are similar to those of mast cells, and they also release the same pharmacologic agents via degranulation. Additionally, basophils also produce and release other pharmacologic agents from the arachidonic acid in their membranes.

•Circulating blood also contains cell fragments known as platelets (thrombocytes). These small, oval-to- round structures, derived from megakaryocytes of the bone marrow, function in hemostasis, the clotting mechanism of blood.

surface markers and surface immunoglobulins (SIGs) on their plasmalemma. They are formed in and become immunocompetent in the bone marrow. They are responsible for the humoral response and, under the direction of TH2 cells and in response to an antigenic challenge, will differentiate into antibody-manufacturing plasma cells and B memory cells.

•Null Cells are of two types, PHSCs and NK cells.

Pluripotential hemopoietic stem cells resemble lymphocytes and are responsible for the formation of all of the formed elements of blood.

NK cells belong to the null cell population. They

possess FC receptors but no cell surface determinants and are responsible for nonspecific cytotoxicity against virus-infected and tumor cells. They also function in antibody-dependent cell-mediated cytotoxicity.

Lymphocytes

The three types of lymphocytes—B lymphocytes (B cells), T lymphocytes (T cells), and null cells—are morphologically indistinguishable. It is customary to speak of T cells as being responsible for the cellularly mediated immune response and B cells as functioning in the humorally mediated immune response. Null cells are few in number, possess no determinants on their cell membrane, and are of two types, pluripotential hemopoietic stem cells (PHSCs) and natural killer (NK) cells.

•T cells not only function in the cellularly mediated immune response but also are responsible for the formation of cytokines that facilitate the initiation of the humorally mediated immune response.

T cells are formed in the bone marrow and migrate to the thymic cortex to become immunocompetent cells. They recognize epitopes (antigenic determinants) that are displayed by cells possessing HLA (human leukocyte antigen; also known as major histocompatibility complex molecules).

There are various subtypes of T cells, each possessing a T-cell receptor (TCR) surface determinant and cluster of differentiation determinants (CD molecules). The former recognizes the epitope, whereas the latter recognizes the type of HLA on the displaying cell surface.

The various subtypes of T cells are memory T cells, T helper cells (TH0, TH1, TH2, and TH17), cytotoxic T cells (CTLs), T regulatory cell (Treg), natural T killer cells, and T memory cells (see Chapter 9, Lymphoid Tissue, for additional information).

Neutrophils

Neutrophils have multilobed nuclei and possess three types of granules—specific granules, azurophilic granules, and tertiary granules.

•Specific granules contain pharmacologic agents and enzymes that permit the neutrophils to perform their antimicrobial roles.

•Azurophilic granules are lysosomes, containing the various lysosomal hydrolases, as well as myeloperoxidase, bacterial permeability increasing protein, lysozyme, and collagenase.

•Tertiary granules contain glycoproteins that are dedicated for insertion into the cell membrane, as well as gelatinase and cathepsins.

Neutrophils use the contents of the three types of granules to perform their antimicrobial function. When neutrophils arrive at their site of action, they exocytose the contents of their granules.

•Gelatinase increases the neutrophil’s capability of migrating through the basal lamina, and the glycoproteins of the tertiary granules aids in the recognition and phagocytosis of bacteria into phagosomes of the neutrophil.

•Azurophilic granules and specific granules fuse with and release their hydrolytic enzymes into the phagosomes, thus initiating the enzymatic degradation of the microorganisms.

In addition to the enzymatic degradation, microorganisms are also destroyed by the capability of neutrophils to undergo a sudden increase in O2 utilization, known as a respiratory burst.