Peripheral blood. Types of peripheral blood cells

Blood is a unique connective tissue in that the extra­cellular matrix forms a circulating fluid compartment called plasma (55% by volume) in which are suspended the cellular elements (45% by volume). The fluid plasma compartment contains proteins (i.e., globulins, albu­mins, fibrinogen, and assorted enzymes), hormones, me­tabolites, various salts in solution, and colloidal sub­stances such as chylomicrons in dispersed form. The extracellular matrix of typical connective tissues usually contains formed connective tissue fibers such as colla­gen or elastin; however, in peripheral blood, fibrous pro­teins are not normally formed unless clotting has initi­ated the polymerization of the soluble monomeric blood protein, fibrinogen, into insoluble fibrin fibers. Serum is equivalent to plasma after removal of fibrinogen and other elements of clotting.

The cellular elements of blood include erythrocytes (red blood cells), leukocytes (white blood cells), and circulating cytoplasmic fragments known as platelets, all of which are formed by hemopoiesis in bone marrow and lymphoid tissues. Red blood cells are anucleate biconcave disks that mainly function within the cardio­vascular system through their efficient transport of oxy­gen and carbon dioxide to and from different tissues/ organs in the body. Platelets are cytoplasmic fragments derived from megakaryocytes, and like red blood cells, they are anucleate. They play an important role in the clotting of blood and also serve as the source of platelet- derived growth factor, an important cytokine in the body.

Two distinct categories of white blood cells have been identified based on their nuclear morphology and the presence of characteristic specific granules within their cytoplasm. The first category, agranular or mono­nuclear leukocytes, includes lymphocytes and monocytes, which possess round or slightly indented nuclei and small lysosomes known as azure granules. Lymphocytes can be subdivided into two important groups based on their function. T lymphocytes are long-lived cells involved in cellular immunity, includ­ing delayed hypersensitivity, graft rejection, and, for some antigens, the modulation of antibody responses. Subsets of T cells regulating these processes are referred to as T helper cells, T suppressor cells, cytotoxic T cells, and memory T cells. В lymphocytes are primarily in­volved in humoral immunity and are responsible, after appropriate stimulation, for the formation in various tis­sues of plasma cells that secrete immunoglobulins.

Monocytes circulate for brief periods in peripheral blood and then migrate into tissues where they differ­entiate into macrophages. Their primary role is the phagocytosis and digestion of microorganisms or sub­stances recognized as nonself, as well as the processing of antigens for stimulation of immunocompetent lym­phoid cells. The monocyte may also give rise to actively phagocytic cells distributed throughout the body, known as the mononuclear phagocytic system (i.e., con­nective tissue macrophages [histiocytes], foreign body giant cells, Kupffer cells, alveolar macrophages, perito­neal macrophages, microglia, osteoclasts, and free or fixed macrophages in lymphoid tissues).

The second category of white blood cells are the gran­ular leukocytes, which have lobulated nuclei and spe­cific granules in their cytoplasm that stain with com­ponents of the Romanovsky-based stains and from which these cells draw their names (neutrophils, eo­sinophils, and basophils). Neutrophils are also known as PMNs (polymorphonuclear cells) because of the presence of two to five nuclear lobes. Specific gran­ules of neutrophils contain lysozyme, lactoferrin, colla­genase, and bactericidal cationic proteins, while the azure granules (lysosomes) found in most white blood cells contain acid hydrolases, myeloperoxidase, lyso­zyme, elastase, and neutral proteases. Neutrophils play a major role in bacterial phagocytosis.

Eosinophils contain a complex group of substances in their specific granules including acid phosphatase, arylsulfatase, histaminase, beta-glucuronidase, cathep- sin, phospholipase, eosinophilic peroxidase, major basic protein, and RNAase. Eosinophils are drawn into con­nective tissue by factors released by mast cells during allergic reactions and parasitic infections. The eosinophil neutralizes agents secreted by mast cells (i.e., leu- kotrine C, histamine), thereby modulating the allergic reaction enhanced by mast cells, and also participates in the phagocytic disposal of antigen-antibody com­plexes formed in allergic responses, such as hay fever or asthma.

Basophils contain specific granules rich in heparin, histamine, chemotactic factors for eosinophils and neu­trophils, leukotrines, platelet-activating factor, and en­zymes capable of degrading connective tissue elements. These cells, like mast cells, have immunoglobin E bound to receptors on their surface, and immunologic trigger­ing of them results in secretion of specific granules lead­ing to immediate hypersensitivity reaction. In its ex­treme form this is referred to as anaphylactic shock.

The differential count in peripheral blood, size, shape, cytoplasmic characteristics, and life span of the different blood cells are shown in the following table.