Chapter 12

• Immunity is a series of delicately balanced, complex, multicellular, and physiologic mechanisms that allow an individual to distinguish foreign material from “self” and to neutralize and/or eliminate that foreign matter.
• Innate immunity, which eliminates most potential pathogens before significant infection occurs, includes physical and biochemical barriers both inside and outside of the body as well as immune cells designed for specific responses.
• Acquired immunity involves producing a specific immune response to each infectious agent (specificity) and remembering that agent so as to mount a faster response to a future infection by the same agent (memory).
• Autoimmunity occurs when the reactions of the immune system are directed against the body's own tissues, resulting in tissue damage and disease.
• Hypersensitivity reactions require prior exposure leading to sensitization in order to elicit a reaction on subsequent challenge.
• Xenobiotics that alter the immune system can upset the balance between immune recognition and destruction of foreign invaders and the proliferation of these microbes and/or cancer cells.

Immunity is a homeostatic process, a series of delicately balanced, complex, multicellular, and physiologic mechanisms that allow an individual to distinguish foreign material from “self” and to neutralize and/or eliminate the foreign matter. Decreased immunocompetence (immunosuppression) may result in repeated, more severe, or prolonged infections as well as the development of cancer. Immunoenhancement may lead to immune-mediated diseases such as hypersensitivity responses, and if some integral bodily tissue is not identified as self, an autoimmune disease may be the end result.

The immune system comprises numerous lymphoid organs and numerous different cellular populations with a variety of functions. The bone marrow and the thymus support the production of mature T and B lymphocytes and myeloid cells, such as macrophages and polymorphonuclear cells (PMN), and are referred to as primary lymphoid organs.

Within the bone marrow, the cells of the immune system developmentally “commit” to either the lymphoid or myeloid lineages. Cells of the lymphoid lineage make a further commitment to become either T cells or B cells. T-cell precursors are programmed to leave the bone marrow and migrate to the thymus, where they differentiate further.

Mature naive or virgin lymphocytes (those T and B cells that have never undergone antigenic stimulation) are first brought into contact with exogenously derived antigens within the spleen and lymph nodes, otherwise known as the secondary lymphoid organs.

Lymphoid tissues associated with the skin and the mucosal lamina propria of the gut, respiratory tract, and genitourinary tract can be classified as tertiary lymphoid tissues. Tertiary lymphoid tissues are primarily effector sites where memory and effector cells exert immunologic and immunoregulatory functions.

### Antigen Recognition

#### Immunity

Innate immunity is a nonspecific first-line defense response with no associated immunologic memory. The innate immune response to a foreign organism is the same for a secondary or tertiary exposure as it is for the primary exposure. ...

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