Blood smear and general staining is the best way of observing and studying the various cellular components of blood. A blood smear is a drop of blood spread over a glass slide, and then fixed and is stained.
Erythrocytes give the red color to the blood for its high content of hemoglobin, a protein which contains iron in its structure. Erythrocyte, in mammals, can be regarded as a modified cell because it has no nucleus and lacks mitochondria and other cellular organelles. It is shaped as a biconcave disc of about 7.5 µm in diameter, which gives greater exchange surface with the blood plasma. Erythrocytes are approximately 45% of blood volume.
Platelets are small portions of cytoplasm without nucleus. The main function of platelets is to cooperate on agglutination and blood coagulation. They are present in mammals, but not in lower vertebrates, and are generated by fragmentation of the cytoplasm of megakaryocytes, a cell type found in the bone marrow.
Leukocytes (white blood cells) contain nucleus and are colorless in fresh blood. Leukocyte cytoplasm contain two types of granules: azurophilic or primary granules, which are lysosomes, and specific or secondary granules, containing diverse types of substances. White blood cells are classified into granular and agranular. They all have azurophilic granules, but the specific granules only appear in the granular leukocytes.
Granular leukocytes encompass neutrophils, eosinophils and basophils, whereas agranular leukocytes include lymphocytes and monocytes. Neutrophils are the most abundant granular leukocytes and account for 60-70% of all leukocytes. They are easily recognized by their multilobed nucleus. Although they show cytoplasmic azurophilic granules, the specific granules are more abundant. Specific granules contain lysozymes, complement activators, collagenases, etcetera. Eosinophils account for 2 to 5% of the leukocyte population. Their nucleus is bilobed and the cytoplasmic specific granules show a strong appetite for acid dyes such as eosin. Basophils are the less abundant and smaller granular leukocytes, and are up to 0.5% of the total leukocytes. Their nucleus is slightly lobed. They contain specific granules which are stained by basic dyes such as haematoxylin.
Agranular leukocytes lack specific granules in their cytoplasm, but have a small population of nonspecific grains. Lymphocytes are the second most abundant type of leukocyte, after neutrophils, and account for 20 to 35% of blood cells. These cells are small, although they show some variability in size, which appears not to be related to the different types of lymphocytes. The two main groups of lymphocytes are B and T. The other agranular leukocytes are monocytes. Monocytes show large size in blood smears and have a kidney-shaped nucleus. Monocytes help in the defense of the body by leaving the blood and moving to the site of infection or injury, where they become macrophages.