The Immune System: Innate and Adaptive Defenses

This set of vocabulary flashcards covers the fundamental components and processes of the immune system, including innate barriers, phagocytosis, inflammatory response, lymphocyte maturation, and adaptive humoral and cellular defenses.

Innate (nonspecific) defense system

The intrinsic system that constitutes the first and second lines of defense, providing immediate resistance to disease.

First line of defense

External body membranes, specifically the skin and mucosae, that act as surface barriers to most microorganisms.

Second line of defense

Internal defenses consisting of antimicrobial proteins, phagocytes, and cells that inhibit the spread of invaders via processes like inflammation and fever.

Adaptive (specific) defense system

The third line of defense that attacks particular foreign substances and takes longer to react than the innate system.

Pathogens

Disease-causing microorganisms.

Keratin

A protein in the skin resistant to weak acids and bases, bacterial enzymes, and toxins.

Acid mantle

The acidity of skin and some mucous secretions that inhibits the growth of microorganisms.

Lysozyme

An enzyme found in saliva, respiratory mucus, and lacrimal fluid that kills many microorganisms.

Mucin

Sticky mucus lining the digestive and respiratory tracts that traps microorganisms.

Defensins

Antimicrobial peptides that inhibit microbial growth.

Phagocytes

White blood cells that ingest and digest foreign invaders.

Neutrophils

The most abundant phagocytes that become phagocytic upon exposure to infectious material but die fighting.

Macrophages

The chief phagocytic cells that develop from monocytes and are more robust than neutrophils.

Stellate macrophages

Fixed macrophages that are permanent residents of the liver.

Microglia

Fixed macrophages that are permanent residents of the brain.

Opsonization

The process where the immune system uses antibodies or complement proteins to coat pathogens, acting as handles for phagocytes to grab.

Phagosome

A vesicle formed when cytoplasmic extensions (pseudopods) bind to and engulf a particle.

Phagolysosome

The structure formed by the fusion of a phagosome with a lysosome, where enzymes digest the ingested particle.

Natural Killer (NK) Cells

Large granular lymphocytes that police blood and lymph, killing cancer and virus-infected cells by inducing apoptosis.

Apoptosis

Programmed cell death induced by NK cells and cytotoxic T cells.

Perforins

Chemicals released by NK cells that polymerize and form holes in the enemy cell membrane.

Granzymes

Enzymes from NK cells that enter perforin holes to degrade enemy cell enzymes.

Inflammation

A response triggered by tissue injury used to prevent damage spread, dispose of debris, and alert the adaptive immune system.

Cardinal signs of acute inflammation

Redness, heat, swelling, and pain.

Histamine

A key inflammatory chemical released by basophils and mast cells that causes vasodilation and increased capillary permeability.

Hyperemia

Congestion with blood that leads to redness and heat during inflammation.

Exudate

Fluid containing clotting factors and antibodies that leaks from capillaries into tissue during inflammation.

Leukocytosis

The first step of phagocyte mobilization where neutrophils enter the blood from red bone marrow.

Margination

The process where neutrophils cling to the capillary wall in response to inflammatory chemical signals.

Diapedesis

The process where neutrophils flatten and squeeze out of capillaries into the tissue.

Chemotaxis

The process where neutrophils follow a chemical trail to the site of inflammation.

Interferons (IFN)

Immune modulating proteins secreted by virus-infected cells that warn neighboring cells to block viral reproduction.

Complement system

A group of approximately $20$ blood proteins that, when activated, enhance inflammation and directly destroy bacteria via cell lysis.

Classical pathway

Complement activation pathway triggered when antibodies first bind to invading organisms.

Membrane attack complex (MAC)

A complex of complement proteins ($C5b$ and $C6-C9$) that inserts into a target cell membrane, creating pores and causing lysis.

Pyrogens

Chemicals secreted by leukocytes and macrophages that act on the hypothalamus to raise body temperature.

Humoral immunity

Antibody-mediated immunity produced by B lymphocytes targeting extracellular bacteria, toxins, and free viruses.

Cellular immunity

Cell-mediated immunity involving T lymphocytes targeting intracellular virus-infected cells, cancer cells, and foreign grafts.

Antigens

Large, complex molecules not normally found in the body that can mobilize adaptive defenses and provoke an immune response.

Immunogenicity

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

Reactivity

The ability of an antigen to react with activated lymphocytes and released antibodies.

Haptens

Small molecules (incomplete antigens) that are reactive but not immunogenic unless they attach to the body's own proteins.

Antigenic determinants

Specific parts of an antigen molecule that antibodies or lymphocyte receptors bind to.

MHC proteins

Self-antigen glycoproteins coded by the Major Histocompatibility Complex that present antigen fragments to T cells.

Immunocompetence

The ability of a lymphocyte to recognize only one specific antigen by displaying a unique receptor on its surface.

Self-tolerance

The property of lymphocytes being unresponsive to the body's own antigens.

Positive selection

The first test in T cell maturation that selects T cells capable of recognizing self-MHC proteins.

Negative selection

The process of clonal deletion that eliminates T cells that bind to self-antigens, ensuring self-tolerance.

Clonal selection

The process where a naive lymphocyte is selected to differentiate into an active cell by binding to its specific antigen.

Plasma cells

Effector B cells that secrete specific antibodies at a rate of $2000$ molecules per second.

Primary immune response

The initial exposure to an antigen characterized by a lag period of $3$ to $6$ days and peak antibody levels at $10$ days.

Secondary immune response

Re-exposure to the same antigen resulting in a faster, more prolonged, and more effective response with higher antibody affinity.

Active humoral immunity

Immunity acquired when B cells encounter antigens and produce antibodies, either naturally from infection or artificially from vaccines.

Passive humoral immunity

Immunity acquired when ready-made antibodies are introduced (e.g., via placenta or injection), providing no immunological memory.

Neutralization

The defensive mechanism where antibodies block specific sites on viruses or toxins to prevent them from binding to tissue cell receptors.

Agglutination

The process where antibodies cross-link cell-bound antigens into large lattice-like clumps.

Precipitation

The cross-linking of soluble antigen molecules into complexes that fall out of solution.

CD4 cells

T cells that usually become helper T cells ($T_H$) or regulatory T cells.

CD8 cells

T cells that become cytotoxic T cells ($T_C$) capable of destroying cells harboring foreign antigens.

Class I MHC proteins

MHC proteins found on all nucleated cells that present endogenous antigens and are recognized by CD8 cells.

Class II MHC proteins

MHC proteins found only on APCs that present exogenous antigens and are recognized by CD4 cells.

MHC restriction

The requirement that CD4 cells bind only to Class II MHC and CD8 cells bind only to Class I MHC proteins.