Chapter 21: The Immune System

Comprehensive vocabulary flashcards covering Chapter 21: The Immune System, including innate defenses, phagocytosis, inflammation, antimicrobial proteins, humoral immunity, antibody classes and mechanisms, cell-mediated immunity, T-cell education, graft types, immunodeficiencies, and hypersensitivities.

Innate (Nonspecific) Defense System

The intrinsic system consisting of surface barriers (skin and mucous membranes) as the first line of defense, and internal defenses (antimicrobial proteins, phagocytes, natural killer cells, fever, and inflammation) as the second line of defense.

Adaptive (Specific) Defense System

The third line of defense that targets specific foreign substances, is systemic, possesses immunological memory, and consists of humoral (B-cell) and cellular (T-cell) immunity.

Acid Mantle

The acidic surface film produced by skin secretions that inhibits bacterial growth.

Lysozyme

An enzyme present in saliva, respiratory mucus, and lacrimal fluid that destroys bacteria by breaking down cell walls.

Defensins

Antimicrobial peptides produced by mucous membranes and neutrophils that inhibit microbial growth and pierce pathogen membranes.

Neutrophils

The most abundant white blood cells that become actively phagocytic upon encountering infectious material and die fighting at the infection site.

Macrophages

The chief phagocytic cells of the immune system that differentiate from circulating monocytes and exist as either free or fixed tissue residents.

Kupffer Cells

Fixed macrophages that reside permanently within liver tissue.

Microglia

Fixed macrophages that reside permanently within the brain.

Opsonization

The process of coating pathogens with complement proteins or antibodies to accelerate and facilitate adherence by phagocytes.

Phagolysosome

A cytoplasmic body formed by the fusion of a phagosome (phagocytic vesicle) with a lysosome, wherein acid hydrolase enzymes digest the engulfed microbe.

Respiratory Burst

A pathogen-killing metabolic event stimulated by helper T cells that produces cell-killing free radicals, oxidizing chemicals like $\text{H}_2\text{O}_2$, and elevates the pH and osmolarity inside the phagolysosome.

Natural Killer (NK) Cells

Nonphagocytic large granular lymphocytes of the innate system that destroy cancer and virus-infected cells lacking 'self' surface markers by inducing apoptosis.

Cardinal Signs of Acute Inflammation

The clinical hallmarks of acute inflammation: redness, heat, swelling, and pain (with impairment of function sometimes considered a fifth sign).

Edema

Local tissue swelling caused by increased capillary permeability that allows protein-rich exudate containing clotting factors and antibodies to seep into interstitial spaces.

C-Reactive Protein (CRP)

A clinical marker protein synthesized by the liver in response to inflammatory molecules, used to monitor acute infection, inflammatory conditions, and treatment efficacy.

Leukocytosis

The first step of phagocyte mobilization wherein neutrophils are released from the red bone marrow into the bloodstream in response to leukocytosis-inducing factors from injured cells.

Margination

The second step of phagocyte mobilization in which neutrophils cling to inner capillary walls in an inflamed region via cell adhesion molecules (CAMs).

Diapedesis

The third step of phagocyte mobilization wherein neutrophils flatten and squeeze through capillary endothelial walls into the damaged tissue.

Chemotaxis

The directed migration of phagocytes and other leukocytes along a chemical gradient toward the site of tissue injury.

Interferons (IFNs)

A group of cytokine proteins secreted by virus-infected cells that stimulate neighboring cells to synthesize antiviral proteins that block viral reproduction and degrade viral RNA.

IFN Gamma

An immune interferon secreted by lymphocytes that exerts widespread immune-mobilizing effects and directly activates macrophages.

Complement System

A group of roughly $20$ circulating blood proteins that amplify the inflammatory response, promote phagocytosis via opsonization, and destroy target microbes via membrane lysis.

Pyrogens

Fever-inducing chemicals released by leukocytes and macrophages exposed to foreign matter that act on the hypothalamus to elevate body temperature.

Immunogenicity

The ability of an antigen to stimulate the proliferation and clonal expansion of specific lymphocytes.

Reactivity

The capacity of an antigen to interact specifically with activated lymphocytes and antibodies generated against it.

Haptens (Incomplete Antigens)

Small molecules (such as peptides, nucleotides, and hormones) that lack immunogenicity on their own but can trigger a hypersensitivity reaction when bound to body proteins.

Antigenic Determinants

The specific immunogenic regions of an antigen molecule to which antibodies and lymphocyte antigen receptors bind.

Class I MHC Proteins

Surface glycoproteins found on virtually all nucleated body cells (all cells except red blood cells) that present antigens to $\text{CD8}$ cytotoxic T cells.

Class II MHC Proteins

Surface glycoproteins found specifically on antigen-presenting cells (dendritic cells, macrophages, and B cells) that present antigens to $\text{CD4}$ helper T cells.

Antigen-Presenting Cells (APCs)

Specialized immune cells—including dendritic cells, macrophages, and B cells—that internalize antigens and display fragments on their surface for T-cell recognition.

Dendritic Cells

The most effective antigen-presenting cells known, located in connective tissues and the epidermis, which engulf pathogens and migrate to lymph nodes to activate naive T cells.

Immunocompetence

The capability of a mature lymphocyte to recognize and bind to a specific foreign antigen using its unique surface receptors.

Self-Tolerance

The state of lymphocyte unresponsiveness to the body's own self-antigens, preventing autoimmune destruction.

Positive Selection

The thymic process that selects developing T cells capable of recognizing self-MHC proteins (MHC restriction), consigning those that fail to apoptosis.

Negative Selection

The thymic process that triggers apoptosis in developing T cells that bind tightly to self-antigens displayed by self-MHC, ensuring self-tolerance (survived by only about $2\%$ of T cells).

Anergy

A state of unresponsiveness or inactivation in self-reactive B cells that escape elimination in the bone marrow.

Plasma Cells

Effector B cells that synthesize and secrete antibodies at a rate of approximately $2000$ molecules per second for $4\text{ to }5\text{ days}$ before dying.

Memory Cells

Long-lived clone cells generated during primary lymphocyte activation that provide immunological memory and initiate rapid, intense secondary responses upon re-exposure.

Primary Immune Response

The initial adaptive immune response to an antigen, characterized by a $3\text{ to }6\text{ day}$ lag period, with peak plasma antibody titers attained around day $10$.

Secondary Immune Response

A faster, more prolonged, and higher-affinity antibody response upon re-exposure to a known antigen, wherein memory cells respond within hours and antibody titers peak within $2\text{ to }3\text{ days}$.

Active Humoral Immunity

Immunity established when B cells encounter an antigen and manufacture antibodies, occurring naturally via infection or artificially through vaccination.

Passive Humoral Immunity

Short-lived immunity provided by introducing exogenous antibodies into the body without antigenic challenge or memory formation, occurring naturally (placental or milk transfer) or artificially (gamma globulin injection).

IgM

The first class of antibody released during a primary immune response; a potent agglutinating agent with high avidity that readily fixes and activates complement.

Secretory IgA

The antibody class found in mucous secretions, saliva, tears, and breast milk (colostrum) that prevents pathogens from adhering to mucosal surfaces.

IgD

A monomeric antibody class anchored to the surface of B cells that functions as an antigen receptor.

IgG

The most abundant antibody class in plasma ($75\text{--}85\%$), dominant in secondary and late primary responses, and the only class capable of crossing the placenta.

IgE

An antibody monomer involved in Type I hypersensitivity reactions (prompting histamine release from mast cells and basophils) and defenses against parasitic worms.

PLAN

The acronym summarizing the defensive actions of antibodies: Precipitation, Lysis (by complement), Agglutination, and Neutralization.

Neutralization

An antibody mechanism where antibodies mask specific sites on bacterial exotoxins or viruses, preventing them from binding to cellular receptors.

Agglutination

The clumping of foreign, cell-bound antigens caused by cross-linking multiple cells with multi-binding-site antibodies.

Precipitation

An antibody mechanism wherein soluble antigen molecules are cross-linked into large insoluble complexes that fall out of solution and undergo phagocytosis.

CD4 Cells

T cells bearing the $\text{CD4}$ glycoprotein that typically differentiate into helper T ($\text{T}_{\text{H}}$) cells or regulatory T ($\text{T}_{\text{Reg}}$) cells upon binding antigens presented on Class II MHC proteins.

CD8 Cells

T cells bearing the $\text{CD8}$ glycoprotein that differentiate into cytotoxic T ($\text{T}_{\text{C}}$) cells upon recognizing antigens presented on Class I MHC proteins.

Interleukin 1 (IL-1)

A cytokine released by macrophages that co-stimulates antigen-bound T cells to release interleukin 2 ($\text{IL-2}$) and express more $\text{IL-2}$ receptors.

Interleukin 2 (IL-2)

A key T-cell growth factor cytokine that promotes the rapid division of activated T cells and is used clinically to enhance immune responses against melanoma and kidney cancer.

Helper T (TH) Cells

Master regulatory T lymphocytes that activate B cells and CD8 cells, induce proliferation of lymphocytes, and recruit additional immune cells via cytokines.

Cytotoxic T (TC) Cells

Effector T lymphocytes that directly bind and kill virus-infected, cancerous, intracellularly parasitized, or foreign cells using perforins and granzymes or cell-death receptors.

Perforins

Pore-forming proteins released by cytotoxic T cells and NK cells that insert into target cell membranes to create transmembrane channels.

Granzymes

Proteolytic enzymes released by cytotoxic T cells and NK cells that enter target cells through perforin-created pores to trigger apoptotic cell death.

Regulatory T (TReg) Cells

Specialized T cells that suppress the immune response through cell contact or inhibitory cytokines to prevent autoimmune disease in peripheral tissues.

Autografts

Tissue grafts transplanted from one anatomical site to another within the same person.

Isografts

Tissue grafts transplanted between genetically identical twins.

Allografts

Tissue grafts transplanted between genetically distinct individuals of the same species.

Xenografts

Tissue grafts transplanted from an animal donor of one species into a recipient of another species.

Severe Combined Immunodeficiency (SCID) Syndrome

A congenital genetic defect characterized by marked deficits in both B and T cells, frequently caused by a defective adenosine deaminase (ADA) enzyme.

Hodgkin's Disease

An acquired immunodeficiency resulting from a malignancy of B cells that depresses lymph node cellularity and function.

Human Immunodeficiency Virus (HIV)

A retrovirus transmitted by infected body fluids that destroys helper T ($\text{T}_{\text{H}}$) cells, causing acquired immune deficiency syndrome (AIDS).

Anaphylactic Shock

A severe systemic Type I hypersensitivity response resulting in massive histamine release, acute bronchiole constriction, extreme vasodilation, fluid loss, and circulatory collapse.

Cytotoxic (Type II) Hypersensitivities

Subacute hypersensitivities occurring within $1\text{ to }3\text{ hours}$ where IgM or IgG antibodies bind to cellular antigens, stimulating phagocytosis and complement-mediated lysis (e.g., mismatched blood transfusions).

Immune Complex (Type III) Hypersensitivities

Subacute hypersensitivities where insoluble antigen-antibody complexes settle in tissues, resisting clearance and causing intense inflammation, cell lysis, and neutrophil damage (e.g., SLE, glomerulonephritis, farmer's lung).

Delayed (Type IV) Hypersensitivities

Cell-mediated hypersensitivities with a slow onset of $1\text{ to }3\text{ days}$, driven by cytokine-activated macrophages and cytotoxic T cells (e.g., allergic contact dermatitis and the TB skin test).