Communicable diseases and infections

One way to combat communicable diseases and infections is to stimulate the body’s own immune system to resist the unwanted microorganisms. In general, this is called immunomodulation. Specifically, when an aug- mented or enhanced immune response is required, it is known as biological response modification. The mechanism of immunomodulation is complex, and a rather daunting nomenclature has grown up around the various components and factors involved.

The immune system can be separated into two functional divisions. One is innate immunity, which involves the stimulation of cells that non- specifically recognize foreign invaders and destroy them. The other is adaptive immunity, which differs from the innate immune response in two critical areas: specificity and memory. A clear distinction of the adaptive division from the innate division is not possible because both divisions heavily interact.

The major effector cells of the innate system are illustrated in Figure 11.1 . They include large granular lymphocytes, also called natural killer (NK) cells. They are ready to kill a variety of target cells such as tumor cells or viral-infected host cells as soon as they are formed rather than requiring the maturation and activation process required by the cells of the adaptive immunity process. Polymorphonuclear leukocytes consti- tute the primary human defense against pathogenic bacteria and migrate to sites of infection in a process termed chemotaxis. These effector cells are capable of phagocytizing and killing microorganisms without help; how- ever, they function more efficiently when pathogens are first opsonized (coated) by components of the complement system or by antibodies.

Macrophages and monocytes are mononuclear cells of the innate immune system capable of phagocytosis. They also have the ability to release soluble factors with inflammatory properties. Monocytes are found circulating in the bloodstream, and the macrophages are fixed to the tissue of the reticuloendothelial system, located in the liver, spleen, GI tract, lymph nodes, brain, and others. Both monocytes and macrophages act as antigen-presenting cells to stimulate the adaptive immune system. Phagocytosis by these cells results in the digestion of the infecting organism into small peptide fragments that are antigenic. The antigenic fragments are combined with major histocompatibility complex (MHC) proteins on the surface of the antigen-presenting cell, where they are

212 Tyler's herbs of choice: The therapeutic use of phytomedicinals

Innate (Nonspecific) Immune System

Polymorphonuclear

Large granular leukocytes

Circulating

Fixed tissue

lymphocytes Chemotaxis

(natural killer cells)

Pathogen

Tumor cells, Viral infected host cells

Phagocytosis

Formation of antigen/MHC

Cell lysis

complex

Cell lysis

Antigen-presenting cells

Adaptive (Specific) Immune System Cellular

Activated

Immunity

T lymphocytes

Helper T lymphocytes Activation B lymphocytes

Activation

Humoral Immunity

Plasma cells

Antibodies

Figure 11.1 Schematic model of intracellular interactions between effector cells of the immune system.

recognized by the T cell receptors on the surface of T lymphocytes. MHC constitutes a group of molecules important in helping the body distin- guish self from nonself.

The main types of effector cells of the adaptive immune system are T lymphocytes (T cells), which protect against intracellular diseases and cellular neoplasms, and B lymphocytes, which develop into plasma cells that secrete antibodies or immunoglobulins in response to certain antigens. The T lymphocytes are classified into three types, depending on their function: T helper cells secrete lymphokines that stimulate the activity of T killer cells, T suppressor cells control numerous immune reactions, and T killer cells produce tumor necrosis factor.

Chapter eleven: Performance and immune deficiencies 213 In summary, T lymphocytes produce cellular immune responses, and

plasma cells derived from B lymphocytes, by the production of antibodies, induce humoral immune responses. Macrophages secrete so-called cytokines that influence all immune responses; lymphocytes communi- cate through the secretion of lymphokines. 64

Biological response modifiers (BRMs) or immunostimulants may affect the cellular or humoral immune system or both. They are nonspecific in character, producing general stimulation of the entire system. Because the response capacity of the immune system is limited, BRMs are more effective when used in conjunction with other chemotherapeutic agents and when the disease entity is quantitatively small.

Communicable diseases and infections