Color Atlas of Hematology - Practical Microscopic and Clinical ...

48Normal Cells of the Blood and Hematopoietic OrgansLymphocytes (and Plasma Cells)Lymphocytes are produced everywhere, particularly in the lymph nodes,spleen, bone marrow, and the lymphatic islands of the intestinal mucosa,under the influence of the thymus (T-lymphocytes, about 80%), or thebone marrow (B-lymphocytes, about 20%). A small fraction of the lymphocytesare NK cells (natural killer cells). Immature precursor cells (lymphnode cytology, p. 177) are practically never released into the blood and aretherefore of no practical diagnostic significance. The cells encountered incirculating blood are mostly “small” lymphocytes with oval or round nuclei6–9 µm in diameter. Their chromatin may be described as dense andcoarse. Detailed analysis under the microscope, using the micrometerscrew to view the chromatin in different planes, reveals not the patch-likeor banded structure of myeloblast chromatin, or the “busy” structure ofmonocyte chromatin, but slate-like formations with homogeneous chromatinand intermittent narrow, lighter layers that resemble geologicalbreak lines. Nucleoli are rarely seen. The cytoplasm wraps quite closelyaround the nucleus and is slightly basophilic. Only a few lymphocytes displaythe violet stained stippling of granules; about 5% of small lymphocytesand about 3% of large ones. The family of large lymphocytes withgranulation consists mostly of NK cells. An important point is that smalllymphocytes—which cannot be identified as T- or B-lymphocytes on thebasis of morphology—are not functional end forms, but undergo transformationin response to specific immunological stimuli. The final stage of B-lymphocyte maturation (in bone marrow and lymph nodes) is plasmacells, whose nuclei often show radial bars, and whose basophilic cytoplasmlayer is always wide. Intermediate forms (“plasmacytoid” lymphocytes)also exist.Diagnostic Implications. Values between 1500 and 4000/µl and about 35%reflect normal output of the lymphatic system. Elevated absolute lymphocytecounts, often along with cell transformation, are observed predominantlyin viral infections (pp. 67, 69) or in diseases of the lymphatic system(p. 75 ff.). Relative increases at the expense of other blood cell series maybe a manifestation of toxic or aplastic processes (agranulocytosis, p. 87;aplastic anemia, p. 148), because these irregularities are rare in the lymphaticseries. A spontaneous decrease in lymphocyte counts is normallyseen only in very rare congenital diseases (agammaglobulinemia [Brutondisease], DiGeorge disease [chromosome 22q11 deletion syndrome]).Some systemic diseases also lead to low lymphocyte counts (Hodgkin disease,active AIDS).Mature plasma cells are rarely found in blood (plasma cell leukemia isextremely rare). Plasma-cell-like (“plasmacytoid”) lymphocytes occur inviral infections or systemic diseases (see p. 68 f. and p. 74 f.).