As mentioned in the text, about 96% of total hemoglobin in adults is represented by hemoglobin type HbA. Approx-imately 8% of HbA consists of several subtypes that dis-play slight chemical differences. These subtypes are hemoglobins HbA1a1, HbA1a2, HbA1b, and HbA1c. Of these subtypes, hemoglobin type A1c is of clinical sig-nificance because it binds irreversibly to glucose. It is re-ferred to as glycated or glycosylated hemoglobin. Levels of this hemoglobin subtype are used to monitor an individual’s blood glucose levels over the previous 2 to 3 months (clinically referred to as an A1c test). Individuals with diabetes have increased levels of glycated hemoglobin HbA1c in the blood because of their elevated blood glu-cose. Since the normal life span of erythrocytes is about 120 days (see page 295), glycated hemoglobin can only be eliminated when the RBCs containing it are de-stroyed. Thus, HbA1c values are directly proportional to the concentration of glucose in the blood over the entire life span of the erythrocyte. In healthy individuals and in those with diabetes that is being effectively controlled, HbA1c levels should not be higher than 7% of the total hemoglobin. Since HbA1c values are not subject to the short-term fluctuations in blood glucose levels that are seen, for example, after meals or during fasting, blood for HbA1c testing can be obtained without regard to when food is consumed.
FIGURE 10.4  Structural diagram of the hemoglobin molecule. Each hemoglobin molecule is composed of four subunits. Each subunit contains a heme, the iron-containing portion of hemoglobin, embedded in a hydrophobic cleft of a globin chain. The folding of the globin chain places the heme near the surface of the molecule, where it is readily accessible to oxygen. There are four different types of globin chains: ,  , , and occurring in pairs. The types of globin chains present in the molecules determine the type of hemoglobin. The figure illustrates hemoglobin A (HbA), which is composed of two  and two  chains.
Leukocytes are subclassified into two general groups. The basis for this division is the presence or absence of prominent specific granules in the cytoplasm. As previously noted, cells containing specific granules are classified as granulocytes (neutrophils, eosinophils, and basophils) (Plate 17, page 302), and cells that lack specific granules are classified as agranulocytes (lymphocytes and monocytes) (Plate 18, page 304). However, both agranulocytes and granulocytes possess small number of nonspecific azurophilic granules, which are lysosomes. The relative number of the various leukocytes is given in Table 10.1.
Neutrophils are the most numerous WBCs as well as the most common granulocytes.
Neutrophils measure 10 to 12 m in diameter in blood smears and are obviously larger than erythrocytes. Although named for their lack of characteristic cytoplasmic staining, they are also readily identified by their multilobal nucleus; thus, they are also called polymorphonuclear neutrophils or polymorphs. Mature neutrophils possess two to four lobes of nuclear material joined by thinner nuclear strands (Plate 17, page 302). The arrangement is not static; rather, in living neutrophils the lobes and connecting strands change their shape, position, and even number.