Introduction to the Immune System, Pathogens, and Lymphatic Organs

Vocabulary flashcards covering key terms on pathogens, immune functions, and lymphoid organs introduced in the lecture.

Immune system

Body network that recognizes and responds to harmful external (pathogens) and internal (cancer) factors.

Pathogen

Disease-causing microorganism such as viruses, bacteria, fungi, protozoa, or helminths.

Virus

Acellular microbe that lacks energy production and replicates only inside host cells.

Capsid

Protein coat surrounding a viral genome; together with nucleic acid forms the nucleocapsid.

Viral envelope

Host-derived lipid bilayer containing viral glycoproteins that aid in cell entry via membrane fusion.

+ssRNA virus

Virus whose single-stranded RNA genome functions like eukaryotic mRNA (e.g., Coronaviridae).

Retrovirus

Enveloped RNA virus that uses a DNA step in replication (e.g., HIV).

Gram-positive bacteria

Prokaryotes that retain Gram stain due to thick peptidoglycan wall; lack outer LPS membrane.

Gram-negative bacteria

Bacteria that do not retain Gram stain; possess an outer membrane containing lipopolysaccharide.

Lipopolysaccharide (LPS)

Endotoxin in Gram-negative outer membrane that induces cytokine release, fever, and shock.

Endotoxin

LPS component of Gram-negative cell wall; heat-stable, low potency, strongly pyrogenic.

Exotoxin

Secreted bacterial protein toxin that is highly potent and often enzyme-like (e.g., cholera toxin).

Toxoid

Inactivated or modified toxin that has lost toxicity but retains antigenicity for vaccine use.

Fungus

Eukaryotic organism; includes yeasts, moulds (hyphae), and dimorphic species.

Yeast

Unicellular fungus that reproduces asexually by budding (e.g., Candida albicans).

Hypha

Multicellular filament of moulds; can reproduce via spores and form mycelium.

Dimorphic fungus

Fungus able to switch between yeast and hyphal form depending on environment.

Protozoa

Unicellular eukaryotic parasites classified by motility (e.g., Plasmodium, Giardia).

Helminth

Multicellular parasitic worm, including flukes, tapeworms, and roundworms.

Trematode

Leaf-shaped fluke helminth, e.g., Fasciola hepatica.

Cestode

Segmented hermaphroditic tapeworm, e.g., Taenia species.

Nematode

Bisexual roundworm, e.g., Ascaris lumbricoides.

Complement

Plasma protein cascade that enhances phagocytosis, inflammation, and pathogen lysis.

Natural Killer (NK) cell

Innate lymphocyte that kills virus-infected or transformed cells without prior sensitization.

Primary lymphoid organ

Site where lymphocytes mature (bone marrow for B cells, thymus for T cells).

Secondary lymphoid organ

Sites where mature lymphocytes encounter antigen (lymph nodes, spleen, MALT).

Bone marrow

Primary organ that produces all blood cells and deletes self-reactive B cells.

Thymus

Primary organ for T-cell maturation, positive (MHC restriction) and negative (self-tolerance) selection.

Spleen

Secondary organ; white pulp activates lymphocytes, red pulp removes old RBCs and stores platelets.

Lymph node

Encapsulated secondary organ where antigens activate naive B and T cells; cortex, paracortex, medulla zones.

Mucosa-Associated Lymphoid Tissue (MALT)

Diffuse lymphoid tissue in mucosal sites (tonsils, Peyer’s patches) that monitors pathogens at interfaces.

Lymph

Clear fluid derived from interstitial fluid; transports leukocytes, nutrients, and pathogens via lymphatic vessels.