Basics of the Immune System

A comprehensive set of vocabulary flashcards summarizing key terms and definitions from the lecture on immune-system structure, innate and adaptive defenses, cellular players, chemical mediators, and immune memory.

External defenses

Physical and chemical barriers such as skin and mucous membranes that prevent pathogens from entering the body.

Skin (epidermis)

A multilayered, keratinized outer barrier that blocks pathogen entry; part of the first line of defense.

Mucous membranes

Moist epithelial linings of body tracts that secrete mucus to trap microbes and contain immune factors.

Internal defenses

Immune responses and cells that act after a pathogen breaches external barriers, including white blood cells and lymphatic organs.

Pathogen

Any disease-causing organism (bacteria, virus, fungus, parasite) that can trigger an immune response.

Antigen

A foreign molecule (often protein or carbohydrate) capable of binding antigen receptors and provoking immunity.

Antigen receptor

Protein on antibodies or immune cells that specifically binds a particular antigen shape.

Antibody

Y-shaped protein produced by B cells that binds antigens and helps eliminate pathogens.

Lymphocyte

White blood cell subtype (B cells and T cells) responsible for adaptive immunity.

B cell

Adaptive lymphocyte that differentiates into plasma and memory cells and mediates humoral immunity.

Plasma cell

Activated B cell that secretes large quantities of antibodies.

Memory cell

Long-lived B or T lymphocyte clone that remains dormant, providing swift response upon re-exposure to the same antigen.

T cell

Adaptive lymphocyte that provides cell-mediated immunity and recognizes antigen with a unique receptor.

Cytotoxic T cell

Activated T cell that kills infected or tumor cells by perforin release or limited phagocytosis.

Self-tolerance

Immune system’s ability to avoid attacking the body’s own antigens.

Autoimmune disease

Disorder in which faulty antigen receptors cause the immune system to attack self-tissues.

Recognition (immune)

Initial binding of an antigen to an antigen receptor that triggers an immune response.

Clonal expansion

Rapid multiplication of a lymphocyte after recognition, producing identical effector and memory cells.

Innate immunity

Built-in, nonspecific defense mechanisms that attack many pathogens equally and quickly.

Adaptive immunity

Specific, learned defense that tailors responses to particular antigens and builds memory.

Phagocyte

White blood cell (e.g., neutrophil, macrophage) that engulfs and digests pathogens.

Phagocytosis

Endocytic process by which a phagocyte engulfs and destroys microbes.

Neutrophil

Most abundant phagocytic leukocyte; first responder that ingests bacteria and debris.

Macrophage

Large phagocyte derived from monocytes that engulfs pathogens and releases cytokines.

Interferon

Antiviral protein messenger released by infected cells and lymphocytes to warn neighboring cells and slow viral replication.

Inflammation

Innate response that dilates vessels, increases permeability, and recruits leukocytes to infection sites.

Histamine

Mast-cell chemical that triggers vasodilation and capillary permeability during inflammation.

Cytokine

Signaling molecule released by immune cells that coordinates immune actions such as chemotaxis.

Heparin

Mast-cell anticoagulant that temporarily inhibits clotting to maintain blood flow during inflammation.

Chemotaxis

Directed movement of cells toward higher concentration of chemical signals (e.g., cytokines).

Diapedesis

Process by which leukocytes squeeze through capillary walls into tissues.

Amoeboid movement

Crawling motion leukocytes use to migrate through tissues after diapedesis.

Agglutination

Antibody-mediated clumping of cells or particles, restricting spread and easing phagocytosis.

Opsonization

Tagging of pathogens by antibodies to enhance recognition and ingestion by phagocytes.

Neutralization

Antibodies block binding sites on viruses or toxins, rendering them harmless.

Perforin

Protein released by cytotoxic T cells that forms pores in target cell membranes, leading to lysis.

Cell-mediated immunity

Adaptive defense carried out by T cells that directly attack infected or abnormal cells.

Humoral immunity

Adaptive defense mediated by antibodies produced by B cells circulating in body fluids.

Active immunity

Immunity gained when one’s own immune system produces antibodies and memory cells after exposure or vaccination.

Passive immunity

Temporary immunity gained by receiving ready-made antibodies (e.g., maternal transfer, antiserum).

Vaccine

Preparation of harmless antigen fragments that elicit active immunity without causing disease.

Fever

Systemic rise in body temperature that enhances immune efficiency and inhibits some microbes.

Acid mantle

Thin, slightly acidic film of sebum on skin that deters microbial growth.

Lysozyme

Enzymatic protein in tears, saliva, sweat, etc., that breaks down bacterial cell walls.

Acidic sebum

Oily skin secretion whose low pH discourages bacterial and fungal proliferation.