Properties and Overview of Immune Responses

Question 1

Immunity

Answer 1

Protection against disease, usually infectious disease, mediated by the cells and tissues that are collectively called the immune system. In a broader sense, immunity refers to the ability to respond to foreign substances, including microbes and noninfectious molecules.

Question 2

Immune system

Answer 2

The molecules, cells, tissues, and organs that collectively function to provide immunity, or protection, against foreign organisms.

Question 3

Immune response

Answer 3

A collective and coordinated response to the introduction of foreign substances in an individual mediated by the cells and molecules of the immune system.

Question 4

Vaccine

Answer 4

A preparation of microbial antigen, often combined with adjuvants, that is administered to individuals to induce protective immunity against microbial infections. The antigen may be in the form of live but avirulent microorganisms, killed microorganisms, purified macromolecular components of a microorganism, or a plasmid that contains a complementary DNA encoding a microbial antigen.

Question 5

Innate immunity

Answer 5

Protection against infection that relies on mechanisms that exist before infection, are capable of a rapid response to microbes, and react in essentially the same way to repeated infections. The innate immune system includes epithelial barriers, phagocytic cells (neutrophils, macrophages), natural killer (NK) cells, the complement system, and cytokines, the latter being largely made by dendritic cells (DCs) and mononuclear phagocytes, that regulate and coordinate many activities of the cells of innate immunity.

Question 6

Adaptive immunity

Answer 6

The form of immunity that is mediated by lymphocytes and stimulated by exposure to infectious agents. In contrast to innate immunity, adaptive immunity is characterized by exquisite specificity for distinct antigens and by long-term and specific memory, manifest as more rapid and vigorous responses on repeated exposure to the same antigen. Adaptive immunity is also called specific immunity or acquired immunity.

Question 7

Antigen

Answer 7

A molecule that binds to an antibody or a T-cell receptor (TCR). Antigens that bind to antibodies include all classes of molecules. Most TCRs bind only peptide fragments of proteins complexed with MHC molecules.

Question 8

Inflammation

Answer 8

A reaction of vascularized tissue to infection or cell injury that involves extravascular accumulation of plasma proteins and leukocytes. Acute inflammation is a common result of innate immune responses, and local adaptive immune responses can also promote inflammation. Although inflammation serves a protective function in controlling infections and promoting tissue repair, it can also cause tissue damage and disease.

Question 9

B lymphocyte

Answer 9

The only cell type capable of producing antibody molecules and therefore the mediator of humoral immune responses. B lymphocytes, or B cells, develop in the bone marrow, and mature B cells are found mainly in lymphoid follicles in secondary lymphoid tissues, in bone marrow, and in low numbers in the circulation.

Question 10

T lymphocyte

Answer 10

The key component of cell-mediated immune responses in the adaptive immune system. T lymphocytes mature in the thymus, circulate in the blood, populate secondary lymphoid tissues, and are recruited to peripheral sites of antigen exposure. They express antigen receptors (TCRs) that recognize peptide fragments of foreign proteins bound to self MHC molecules. Functional subsets of T lymphocytes include CD4+ helper T cells and CD8+ CTLs.

Question 11

Epitopes (determinants)

Answer 11

The specific part of a macromolecular antigen to which and antibody or TCR binds. In the case of a protein antigen recognized by a T cell, an epitope is the peptide portion that binds to an MHC molecule for recognition by the TCR. Synonymous with determinant.

Question 12

Diversity

Answer 12

The existence of a large number of lymphocytes with different antigenic specificities in any individual. Diversity is a fundamental property of the adaptive immune system and is the result of variability in the structures of the antigen-binding sites of lymphocyte receptors for antigens (antibodies and TCRs).

Question 13

Tolerance

Answer 13

Unresponsiveness of the adaptive immune system to antigens, as a result of inactivation or death of antigen-specific lymphocytes, induced by exposure to the antigens. Tolerance to self antigens is a normal feature of the adaptive immune system, but tolerance to foreign antigens may be induced under certain conditions of antigen exposure.

Question 14

Autoimmune diseases

Answer 14

A disease caused by the breakdown of self-tolerance such that the adaptive immune system responds to self antigens and mediates cell and tissue damage. Autoimmune diseases can be caused by immune attack against one organ or tissue (e.g., multiple sclerosis, thyroiditis, or type I diabetes) or against multiple and systemically distributed antigens (e.g., systemic lupus erythematosus).

Question 15

Humoral immunity

Answer 15

The type of adaptive immune response mediated by antibodies produced by B lymphocytes. Humoral immunity is the principal adaptive immune mechanism against extracellular microbes and their toxins.

Question 16

Antibody

Answer 16

A type of glycoprotein molecule, also called immunoglobulin (Ig), produced only by B lymphocytes and plasma cells derived from B cells, which binds antigens, often with a high degree of specificity and affinity. Membrane-bound antibodies serve as antigen receptors that initiate B cell activation. Secreted antibodies perform various effector functions, including neutralizing antigens, activating complement, and promoting leukocyte-dependent destruction of microbes. The basic structural unit of an antibody is composed of two identical heavy chains and two identical light chains. The N-terminal variable regions of the heavy and light chains form the antigen-binding sites, whereas the C-terminal constant regions of the heavy chains of secreted antibodies interact with other molecules in the immune system. Every individual has millions of different antibodies, each with a unique antigen-binding site.

Question 17

Cell-mediated immunity (CMI)

Answer 17

The form of adaptive immunity that is mediated by T lymphocytes and serves as the defense mechanism against various types of microbes that are taken up by phagocytes or infect non-phagocytic cells. Cell-mediated immune responses include CD4+ T cell-mediated activation of phagocytes and CD8+ CTL-mediated killing of infected cells.

Question 18

Active immunity

Answer 18

The form of adaptive immunity that is induced by exposure to a foreign antigen and activation of lymphocytes and in which the immunized individual plays an active role in responding to the antigen. This type contrasts with passive immunity, in which an individual receives antibodies or lymphocytes from another individual who was previously actively immunized.

Question 19

Passive immunity

Answer 19

The form of immunity to an antigen that is established in one individual by transfer of antibodies or lymphocytes from another individual who is immune to that antigen. The recipient of such a transfer can become immune to the antigen without ever having been exposed to or having responded to the antigen. Transplacental transfer of IgG from mother to fetus is a physiologic for of passive immunity essential for health of newborn babies. Examples of therapeutic passive immunity are the administration of human sera or plasm containing antibodies specific for potentially lethal microbial toxins, snake venom, or microbes in individual exposed to those toxins or microbes. Treatments with synthetic monoclonal anti-microbial antibodies have also been developed.

Question 20

Opsonization

Answer 20

The process of attaching opsonins, such as IgG or complement fragments, to microbial surfaces to target the microbes for phagocytosis.

Question 21

Antigen-presenting cell (APC)

Answer 21

A cell that displays peptide fragments of protein antigens, in association with MHC molecules, on its surface and activates antigen-specific T cells. In addition to displaying peptide-MHC complexes, APCs also express costimulatory molecules to optimally activate T lymphocytes.

Question 22

Dendritic cells (DCs)

Answer 22

Bone marrow-derived cells found in epithelial barriers, the stroma of most organs, and lymphoid tissues that are morphologically characterized by thin membranous projections. Many subsets of dendritic cells exist with diverse functions. Classical (conventional) dendritic cells (cDCs) function as innate sentinel cells and become APCs for naive T lymphocytes on activation, and they are important for initiation of adaptive immune responses to protein antigen. Plasmacytoid dendritic cells (pDCs) produce abundant type I IFNs in response to exposure to viruses. Monocyte-derived dendritic cells (MoDCs) are derived from blood monocytes during inflammatory reactions.

Question 23

Naive lymphocyte

Answer 23

A mature B or T lymphocyte that has not previously encountered antigen. When naive lymphocytes are stimulated by antigen, they differentiate into effector lymphocytes, such as antibody-secreting B cells, cytokine-producing helper T cells, and CTLs capable of killing target cells. Naive lymphocytes have surface markers and recirculation patterns that are distinct from those of previously activated lymphocytes. (“Naive” also refers to an unimmunized individual.)

Question 24

Clonal expansion

Answer 24

The approximately 1,000 - 100,000-fold increase in number of lymphocytes specific for an antigen that results from antigen stimulation and proliferation of B and T cells. Clonal expansion occurs in lymphoid tissues and is required to generate enough antigen-specific effector T lymphocytes and plasma cells from rare naive precursors to eradicate infections.

Question 25

Effector cells

Answer 25

The cells that perform effector functions during an immune response, such as secreting cytokines (e.g., helper T cells), killing microbes (e.g., macrophages), killing microbe-infected host cells (e.g., CTLs), or secreting antibodies (e.g., plasma cells).

Question 26

Memory lymphocytes

Answer 26

Memory B and T cells are produced by antigen stimulation oof native lymphocytes and survive in a functionally quiescent state for many years after the antigen is eliminated. Memory lymphocytes mediate rapid and enhanced (i.e., memory or recall) responses to second and subsequent exposure to antigens.

Question 27

Cytokines

Answer 27

Proteins that are produced and secreted by many different cell types, and mediate inflammatory and immune reactions. Cytokines are principal mediators of communication between cells of the immune system.

Question 28

Major histocompatibility complex (MHC)

Answer 28

A large genetic locus (on human chromosome 6 and mouse chromosome 17) that includes the highly polymorphic genes encoding the peptide-binding molecules recognized by T lymphocytes. The MHC locus also includes genes encoding cytokines, molecules involved in antigen processing, and complement proteins.

Question 29

Helper T (Th) cells

Answer 29

The class of T lymphocytes whose main function are to activate macrophages and to promote inflammation in cell-mediated immune responses and to promote B cell antibody production in humoral immune responses. These functions are mediated by secreted cytokines and by T cell CD40 ligand binding to macrophage or B cell CD40. Helper T cells express the CD4 molecule and recognize peptide antigens displayed by class II MHC molecules.

Question 30

Cytotoxic (or cytolytic) T lymphocyte (CTL)

Answer 30

A type of T lymphocyte whose major effector function is to recognize and kill hot cells infected with viruses or other intracellular microbes as well as tumor cells. CTLs usually express CD8 and recognize microbial peptides displayed by class I MHC molecules. CTL killing of infected cells involves delivery of the contents of cytoplasmic granules into the cytosol of infected cells, leading to apoptotic death.

Question 31

Regulatory T cells

Answer 31

A population of T cells that inhibits the activation of other T cells and is necessary to maintain peripheral tolerance to self antigens. Most regulatory T cells are CD4+ and express the (alpha) chain of the IL-2 receptor (CD25), CTLA-4, and the transcription factor (FOXP3).