Transcript
Chapter 27: Determinants and Assessment of Hematologic Function
Chapter Summary
The major solid components of the blood include erythrocytes, leukocytes, and platelets (thrombocytes). All of these cells come from the same pluripotential stem cell but take different committed pathways to maturation.
Erythrocytes provide an effective oxygen transport system. Adequate production of healthy erythrocytes requires erythropoietin, vitamin B12, and folic acid. Hemoglobin, the major component of the erythrocyte, is the actual oxygen carrier. Hemoglobin requires iron as an integral part of its molecular structure. Oxygen molecules attach to the iron.
The purpose of the immune system is to discriminate between what is self (not harmful) and nonself (potentially harmful). The body has several avenues by which it protects itself against foreign antigens (nonself). Innate (natural) immunity is present at birth. It is nonspecific (cannot discriminate between antigens), and species-specific, but lacks antigen memory. Adaptive (acquired) immunity develops following birth and is characterized by antibody protection following exposure to antigens, through long-term antigenic memory.
Blood cells of innate (natural) immunity include three phagocytes: neutrophils, monocyte–macrophages, and NK cells. Neutrophils are small, powerful, granulated polymorphonuclear cells that are the first line of defense. Monocytes, on activation as macrophages, provide the second line of defense. Neutrophils and monocytes are able to leave the vascular system and move to a site of inflammation or infection through the mechanisms of margination, diapedesis, migration, and chemotaxis.
Certain cells, particularly macrophages, act as antigen-presenting cells (APCs), whereby they ingest, digest, and present a fragment of antigen for recognition by the T and B cells. Without adequate macrophage support, the remaining cellular components of the immune system would be severely impaired.
Cytokines are special protein-based chemical messengers of the immune response. Their functions are redundant (overlap) and include other hematopoietic functions outside of the immune system. Some of the major cytokines include interleukins (IL), interferons (INF), granulocyte monocyte colony-stimulating factors (GM-CSF), and tumor necrosis factor (TNF).
Adaptive (acquired) immunity is antigen-specific and is the primary responsibility of the T and B lymphocytes. Specific immune response mechanisms include humoral immunity with the formation of antibodies (immunoglobulins) and cell-mediated immunity with its direct-attacking T cells. Adaptive (acquired) immunity can be either passive (transfer of antibodies from one person to another or from some other source, as with breast milk or gamma globulin injections) or active (following exposure).
Active immunity requires at least two exposures to the same antigen. During the primary (first) exposure, time is required (latency period) in which the antigen is recognized as nonself and specifically identified for antibody formation. On subsequent (secondary) exposure to the identical antigen, memory cells quickly recognize the antigen and initiate a strong antibody defense against it. There are five different immunoglobulins (Ig): IgG (80 percent of all Ig), IgA, IgM, IgD, and IgE.
Cell-mediated immunity is the responsibility of the T lymphocytes (T cells). APCs present an antigen, and T cells destroy it. This form of immunity does not involve antibodies. There are three major types of T cells, each one playing a different role in providing cell-mediated immunity: T helper (TH) cells are the chief orchestrators of immune cell activity; T suppressor (TS) cells shut down the immune response once a foreign substance has been destroyed; and T cytotoxic (TC) cells are responsible for destruction and lysis of infected host cells.
Antigens are substances capable of triggering an immune response. They can be nonself (foreign, originating outside of the body) or they can be self (native, originating within the body). Foreign antigens capable of causing disease are called pathogens.
Histocompatibility antigens are protein-based cell surface antigens that are genetically determined. In humans, they are termed human leukocyte antigens (HLA). HLA proteins are paired (haplotypes) and are coded by major histocompatibility complexes (MHC) located on chromosome 6. The HLA antigens help the immune system distinguish self from nonself. Cell surface proteins that are self are ignored by the immune system, while cell surface proteins that are nonself are attacked and destroyed. This accounts for organ transplant rejection. Identical twins are the perfect HLA match.
Platelets are tiny cell fragments that protect the body’s vascular integrity. They are an integral part of the hemostatic and coagulation functions. Their major activity is the sealing of tears in the blood vessels by forming a platelet plug, thereby stopping bleeding.
They continuously circulate throughout the body in an inactive form. When a vessel is injured, the injured endothelium activates them locally to adhere and aggregate at the site of injury.
Hemostasis refers to prevention of blood loss. Formation of a blood clot is the result of a tightly regulated sequential cascade of events involving platelets and coagulation factors. A classic model of this cascade involves two pathways—the intrinsic and the extrinsic, both of which eventually follow a common pathway to fibrin clot formation. Once a blood clot is formed, it retracts (draws in) and either dissolves (dissolution) through plasmin activity or forms a connective tissue fibrin patch.
There are many measurements available to evaluate blood cells. Common tests that evaluate erythrocytes include the reticulocyte count, MCV, total red cell count, hemoglobin and hematocrit, and erythrocyte sedimentation rate (ESR). Tests that evaluate leukocytes include the WBC count with or without the differential cell count. The WBC count with differential cell count is a more valuable test because it separates out and measures the relative values of each of the WBC component cells, including neutrophils, monocytes, eosinophils, and basophils. The bone marrow aspiration biopsy is performed to confirm a diagnosis or to evaluate the severity of a hematologic problem.
