Porth's Pathophysiology, 9e

Chapter 13 Innate and Adaptive Immunity    281

Adaptive Immunity Adaptive immunity (also called acquired immunity ) refers to immunity that is acquired through previous exposure to infectious and other foreign agents. A defining character- istic of adaptive immunity is the ability not only to distin- guish self from nonself but to recognize and destroy specific foreign agents based on their distinct antigenic properties. The components of the adaptive immune system are the T and B lymphocytes and their products. There are two types of adaptive immune responses, humoral and cell-mediated immunity, that function to eliminate different types of microbes. Humoral immunity is mediated by the B lymphocytes (B cells) and is the principal defense against extracellular microbes and their toxins. The B cells differentiate into ­antibody-secreting plasma cells. The circulating antibodies then interact with and destroy the microbes that are present in the blood or mucosal surfaces. Cell-mediated, or cellular, immunity is mediated by the cytotoxic T lymphocytes (T cells) and functions in the elimination of intracellular pathogens ( e.g., viruses). T cells develop receptors that recognize the viral peptides displayed on the surface of infected cells and then signal destruction of the infected cells.

Lymphocyte

Humoral immunity (B lymphocytes)

Extracellular pathogen

B cell

Plasma cell

Antibody

Cell-mediated immunity (T lymphocytes)

Cytotoxic T cell

MHC-I with viral epitope

TCR

Cell with intracellular pathogen being destroyed by cytotoxic T cell

Cell death

ganism. The innate and adaptive immune responses work in concert with one another to ensure that the homeostasis is maintained. Although cells of both the innate and adaptive immune systems communicate critical information about the invading microbe or pathogen by cell-to-cell contact, many interactions and cellular responses depend on the secretion of chemical mediators in the form of cytokines, chemokines, and CSFs. Cytokines are soluble proteins secreted by cells of both the innate and adaptive immune systems that mediate many of the functions of these cells. Chemokines are cytokines that stimulate the migration and activation of various immune and inflammatory cells. CSFs stimulate the growth and differentiation of bone marrow progenitors of immune cells and play a key role in hematopoiesis.

IN SUMMARY Immunity is the body’s defense against disease and invad- ing microorganisms. Immune mechanisms can be divided into two types: innate and adaptive immunity. Innate immunity is the first line of defense and can distinguish between self and nonself through the recognition of cellu- lar patterns on foreign substances and microbes. Adaptive immunity is part of the second line of defense and involves both humoral and cellular mechanisms that respond to cell-specific substances known as antigens. The adaptive immune response is capable of amplifying and ­sustaining its responses, of distinguishing self from nonself, and finally of memory in that it can recognize the antigen on repeat exposure in order to quickly produce a heightened response on subsequent encounters with the same microor-