18 The immune system
Acquired Immunity: The ability of the human body to develop an extremely powerful specific immunity against most invading agents.
Active Acquired Immunity: Immunity that develops when a person comes into direct contact with a pathogen either by contracting the disease produced by the pathogen or by vaccination against the disease.
Antibody: A globulin molecule with the potential to attack agents that are foreign to the host.
Antigen: A protein, large polysaccharide, or large lipoprotein complex that stimulates the process of acquired immunity.
B Lymphocytes or Bursa-Dependent Cells: Immunocompetent lymphocytes that are named for the preprocessing that occurs in the bursa of Fabricius of birds and is responsible for humoral immunity.
Cellular or Cell-Mediated Immunity: A type of acquired immunity that uses sensitized lymphocytes as the primary defense.
Clone: A group of cells that originate from a single parent cell.
Humoral Immunity: A type of acquired immunity that uses antibodies as the primary defense.
Immunity: The ability of the human body to resist almost all types of organisms or toxins that can damage tissues and organs.
Immunodeficiency Disease: Immunosuppression that results from a deficiency of a single humoral antibody group or from a combined deficiency of both T cell and B cell systems.
Immunosuppression: A state of nonresponsiveness of the immune system to antigenic challenge.
Innate Immunity: General processes in the human body, other than those of acquired immunity, that are responsible for protection against organisms and toxins.
Lymphopenia: Decreased function of the lymphoid organs.
Passive Acquired Immunity: Immunity that results when a person receives immune cells or immune serum produced by someone else.
Phagocytosis: The envelopment and digestion of bacteria or other foreign substances.
Sensitized Lymphocytes: Lymphocytes that are made competent by processing to facilitate immunologic activity, such as attachment to and destruction of a foreign agent.
Stem Cells: Unspecialized cells that give rise to specific specialized cells such as T and B lymphocytes.
T Lymphocytes: Sensitized lymphocytes that are responsible for cellular immunity.
Physical and chemical barriers
The body’s first line of immunologic defense is the mechanical barrier provided by the epithelial surface. Some parts of the epithelium have extensions from their surface, such as the cilia and the mucus in the respiratory system. These extensions provide an additional physical barrier to the entrance of foreign substances and an efficient removal system. Hydrochloric acid, which is thought to have bactericidal action, is secreted in the stomach. As an additional defense, the skin produces chemicals that inactivate bacteria. The surfaces of the boundary tissues also have specific defenses in the form of secretory antibodies. Consequently, surgical incisions, intravenous cannulation, and many other invasive procedures can cause major breaks in the first line of defense. Therefore the perianesthesia nurse should use good aseptic or sterile technique to prevent an overwhelming bacterial invasion through the boundary tissues.
Acquired immunity
Humoral immunity
On the second stimulation by the same antigen, a second response occurs. This secondary response, in which a massive amount of antibody specific to the antigen is produced within 1 or 2 days, lasts for months. The secondary response is more rapid, stronger, and more persistent than the primary response because of the memory cells and clones that are produced by the initial exposure to the antigen. If the T lymphocytes are activated by the same antigen, the T lymphocyte helper cells enhance the response of the B lymphocytes; therefore, because of this cooperative effort, the total number of lymphocytes in the lymphoid tissue increases markedly. On second exposure to an antigen, the same plasma cell can produce the particular antibody needed and can convert from one type of antibody secretion to another as needed. When the specific antibodies from the plasma cells are no longer needed, further production of the antibodies is suppressed by the antibodies themselves or by T lymphocyte suppressor cells (Fig. 18-1).
Immunoglobulins, or antibodies, once secreted by the plasma cells, protect the body against invading agents with the following three mechanisms of action: (1) attacking the antigen; (2) activating the complement system, which results in cell lysis; and (3) activating the immediate hypersensitivity reaction, which localizes the invader and may negate its virulence. More specifically, antibodies can inactivate the invading antigen with precipitation, agglutination, neutralization, or complement fixation. Precipitation occurs when an insoluble antibody forms a complex with a soluble antigen, such as tetanus toxin, and the resulting antigen-antibody complex becomes insoluble and precipitates. When antigens are bound together and react with an antibody, agglutinated aggregates occur. Neutralization is achieved when antibodies cover the toxic sites of an antigenic agent or when antibodies counteract toxins released by bacteria. Rarely are the potent antibodies able to attack a cell membrane directly and cause lysis. However, one of the powerful effects of the binding of the antigen-antibody complex is the activation of complement, which serves to amplify this interaction. More specifically, when IgG or IgM binds to an antigen, the complement system is activated and a cascade system of nine different enzyme precursors (C1 through C9) reacts sequentially. The final result of the activation of the complement system is puncture of the antigen’s cell membrane (cell lysis) and rupture of its cellular agents.
IgA is a small molecule that constitutes approximately 15% of the total immunoglobulins and is present in most body secretions. Secretory IgA is effective against viruses and some bacteria that invade the mucous membranes. Secretory immunity is also mediated by IgA. The secretory antibodies are found on the mucosal surfaces of the oral cavity (saliva), the lungs (sputum), and the intestinal and urogenital tracts and in mammary secretions. This secretory IgA differs from other antibodies in that it has a protein molecule, called a secretory piece, attached to it. IgA activates the complement system through a particular sequence of events called the properdin pathway. The complement system is complex cascade of activations of more than 20 proteins that result in the improved ability of phagocytes’ cell killing.
IgD constitutes about 1% of the total immunoglobulins. The exact function of IgD is unknown. Similar to IgA, IgD is situated in the upper respiratory mucosa and works to activate B lymphocytes. IgD has been described as “an ancestral surveillance system at the interface between immunity and inflammation.”1 IgD has also been suggested for relationships in antibody activity directed toward insulin, penicillin, milk proteins, diphtheria toxoid, thyroid antigens, and the products of abnormal tissue growth.
