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3 little larger, and con- stitute about 20 – 25% of the white blood cells. The medium and large lymphocytes are relatively scarce. In all lymphocytes, the nucleus occupies nearly the whole volume of the cell, and the cytoplasm (which surrounds it) forms a thin shell. The typical monocyte is commonly as large as a large lymphocyte (12 μm), constituting 3 – 8% of the white blood cells. The nucleus is relatively small, eccentric, and oval or kidney-shaped; the cytoplasm is relatively larger in volume than that in lymphocytes. The granular white blood cells or granular leukocytes comprise three varieties: neutro- phils, eosinophils, and basophils. Neutrophils are large granular leukocytes with cytoplasmic granules that stain best with neutral dyes. Eosinophils are granular leuko- cytes having cytoplasmic granules that stain with acid dyes (most commonly, eosin dyes). Basophils are white blood cells with granules that stain with basic dyes and are water-soluble. The structures of neutrophils, eosinophils, and basophils vary somewhat in different species. In humans, the neutrophils make up 65 – 75% of the leukocytes. They are about as large as monocytes, with a highly variable nucleus. The cytoplasm contains numerous minute granules. In certain conditions, the neutrophils leave the blood vessels and wander into the con- nective tissue ground substance, where they may then disinte- grate, releasing their granules. These granules are rich in certain hydrolytic enzymes, which become active and take part in some phases of the defense mechanisms of the body. The gran- ules are considered to be pure lysosomes, which are released from the cell during inflammation and in other conditions. Human eosinophils (also called acidophils) are about the same size as the neutrophils, but they are less numerous, constituting about 1% of the leukocytes. The granules that fill the cytoplasm are larger than those of the neutrophils. The basophils found in humans are about the same size as the other granular leuko- cytes. The nucleus may appear elongated or with one or more constrictions. The functions of the leukocytes while they are circulating in the blood are not known. However, when they leave the blood vessels and enter the connective tissue, they constitute an important part of the body's defense and repair mechanisms. Many of the cells are actively phagocytic and engulf debris and bacteria. Lymphocytes are of two major kinds: T cells and B cells. They are involved in the formation of antibodies and in cellular immunity. Lymphocytes develop into plasma cells, which form antibodies even more effectively than lymphocytes. In addition, lymphocytes and monocytes may develop extra- vascularly into macrophages (which are also phagocytic) and subsequently into fibroblasts. The fibroblasts are important in the formation of new connective tissue in regions of injury. Platelets Blood platelets are small spindle-shaped or rodlike bodies that measure about 3 μm in length. They occur in large num- bers in circulating blood. In suitably stained specimens, they consist of a granular central portion (chromomere) embed- ded in a homogeneous matrix (hyalomere). They change their shape rapidly on contact with injured vessels or foreign surfac- es and take part in clot formation. During this process, numer- ous fibrils of fibrin radiate from the platelets. The platelets are not to be regarded as cells and are thought to be cytoplasmic bits broken off from their cells of origin in the bone marrow— the megakaryocytes (Fig. 4). Plasma Plasma is the residual fluid of blood left after removal of the cellular elements. Serum is the fluid that is obtained after blood has been allowed to clot and the clot has been removed. Serum and plasma differ only in their content of fibrinogen and several minor components (which are, in large part, removed in the clotting process). The major constituents of plasma and serum are proteins. The total protein concentration of human serum is approxi- mately 7 g/ml, and most other mammals show similar levels. Birds, reptiles, amphibians, and most fishes have lower protein concentrations of approximately 3 – 5 g/100 ml. It has been demonstrated that serum protein is a heterogeneous mixture of a large number of constituents. More than 60 protein com- ponents have been identified and characterized. Blood (continued) + ward ' s science + ward ' s science Fig. 3: Illustration showing the differences between normal and sickled red blood cells (RBCs). (A) Normal RBCs flow freely in a blood vessel. The inset shows a cross section of a normal RBC with normal hemoglobin. (B) Abnormal, sickled RBCs block blood flow in a blood vessel. The inset shows a cross section of a sickle cell with abnormal (sickle) hemoglobin forming abnormal stiff rods. (Credit: National Heart, Lung, and Blood Institute)