Applied Microbiology Lecture 10 - The Human Immune Response to Microbes

Vocabulary-style flashcards covering the mammalian host immune system, including innate and adaptive responses, chemical mediators, the complement system, and lymphoid tissues.

Immunity

The ability of the host to resist infection or disease.

Innate (Nonspecific) Immunity

The first line of defence against infection present from birth that responds broadly to microbes and does not generate immunological memory.

Adaptive (Specific) Immunity

Immunity that targets specific pathogens or antigens following exposure or vaccination, characterized by a slower initial response and the generation of immunological memory.

Lysozyme

An enzyme produced by mucous membranes that breaks down bacterial cell walls by targeting peptidoglycan.

Lactoferrin

A substance produced by mucous membranes that binds iron, limiting bacterial growth.

Mucociliary escalator

The process where cilia move mucus and trapped microbes away from the lungs to be removed by coughing, sneezing, or swallowing.

Alveolar macrophages

Phagocytic immune cells located in the alveoli that engulf and destroy microbes reaching the lower respiratory tract.

Gut-associated lymphoid tissue (GALT)

Lymphoid tissue in the gastrointestinal tract that monitors pathogens and activates immune responses.

Paneth cells

Cells located in the intestinal epithelium that produce antimicrobial substances including lysozyme and defensins (also known as cryptdins/cryptins).

Cathelicidins

Linear $\alpha$-helical antimicrobial peptides lacking cysteine residues, produced by neutrophils, respiratory and urogenital epithelial cells, and alveolar macrophages.

Defensins

Antimicrobial peptides produced as precursor proteins; $\alpha$-defensins are produced by neutrophils and Paneth cells, while $\beta$-defensins are produced mainly by epithelial cells.

Histatins

Antimicrobial peptides present in saliva that possess antifungal activity.

Bacteriocins

Antimicrobial peptides produced by normal microbiota that inhibit closely related bacterial species, such as colicins by E. coli and lantibiotics by Gram-positive bacteria.

Opsonisation

A process where microbes are coated by opsonins (e.g., $C3b$ or antibodies) to enhance recognition and ingestion by phagocytes.

Membrane Attack Complex (MAC)

A pore-forming structure composed of $C5b$, $C6$, $C7$, $C8$, and $10\text{--}19$ polymerised molecules of $C9$ that causes cell lysis by disrupting membrane integrity.

Classical pathway

A complement system activation pathway usually triggered by antigen–antibody complexes, linking the complement system to adaptive immunity.

Lectin pathway

A complement activation pathway initiated by mannose-binding lectin (MBL) binding to specific carbohydrates on microbial surfaces.

Alternative pathway

A rapid, nonspecific complement pathway triggered by direct interaction between complement proteins and repetitive structures on microbial surfaces, particularly effective against Gram-negative bacteria.

Cytokines

Soluble proteins or glycoproteins released by immune cells that act as signalling molecules to coordinate communication between different parts of the immune system.

Monokines

Cytokines released by mononuclear phagocytes such as macrophages.

Interleukins

Cytokines released by one leukocyte that act on other leukocytes.

Colony-stimulating factors (CSFs)

Cytokines that stimulate the growth and differentiation of immature leukocytes in the bone marrow.

Neutrophils (PMNs)

The most abundant granulocyte in the blood ($\sim 60\%$ of WBCs) which migrate to sites of infection and kill ingested microbes using lytic enzymes and reactive oxygen metabolites.

Eosinophils

Granulocytes ($\sim 3\%$ of WBCs) that defend against protozoan and helminth parasites and contribute to allergic reactions.

Basophils

Nonphagocytic granulocytes ($\sim 1\%$ of WBCs) that release vasoactive mediators like histamine and serotonin, playing roles in allergies and inflammation.

Mast Cells

Bone marrow-derived cells in connective tissues containing granules with histamine and heparin that respond to infection, injury, and allergens.

Macrophages

Tissue-resident phagocytic cells derived from monocytes that initiate inflammation and possess pattern recognition receptors (PRRs) to recognize pathogens.

Dendritic Cells

Sentinel immune cells with neuron-like appendages that capture and process antigens to present them to T lymphocytes, linking innate and adaptive immunity.

Epitopes

The specific antigenic determinant sites on a molecule recognised by an antibody or T-cell receptor.

Antibody Affinity

The strength of binding between one antibody-binding site and one epitope.

Avidity

The overall strength of binding between an antibody and antigen across multiple binding sites.

Antigen-presenting cells (APCs)

Cells like macrophages and dendritic cells that process and display antigen fragments on their surface bound to MHC molecules to activate T lymphocytes.

Natural Killer (NK) Cells

Large granular, non-phagocytic lymphocytes that destroy virus-infected or malignant cells by releasing perforin and granzymes.

Major Histocompatibility Complex (MHC)

A collection of genes (HLA complex on chromosome 6 in humans) involved in self vs non-self recognition and antigen presentation.

MHC I

Molecules found on almost all nucleated cells that present intracellular antigens to $CD8+$ cytotoxic T cells.

MHC II

Molecules found only on antigen-presenting cells (APCs) that present foreign antigens to activate $CD4+$ helper T cells.

Primary Lymphoid Organs

Sites including the bone marrow and thymus where lymphocytes develop and mature into immunocompetent cells.

Secondary Lymphoid Organs

Organized tissues like the spleen and lymph nodes where mature lymphocytes encounter antigens and initiate immune responses.

Skin-associated lymphoid tissue (SALT)

Diffuse lymphoid tissue containing Langerhans cells and intraepidermal lymphocytes that provides immune surveillance in the skin.

Pathogen-associated molecular patterns (PAMPs)

Conserved microbial structures (e.g., Lipopolysaccharide, Peptidoglycan) recognized by PRRs that are absent from host cells.

Toll-Like Receptors (TLRs)

A major class of pattern recognition receptors that function as signalling receptors on cell surfaces or intracellular membranes to trigger cytokine production and inflammation.

Cytotoxic T lymphocytes (CTLs)

Differentiated $CD8+$ T cells that kill infected or abnormal host cells using lytic enzymes and proteins.

Plasma cells

Differentiated B cells that secrete antibodies to neutralize toxins or viruses and enhance opsonisation.

Superantigens

Proteins that bypass normal antigen specificity by directly linking MHC class II and T-cell receptors, causing massive cytokine release or a "cytokine storm."

Fab Region

The portion of an antibody containing variable regions that specifically binds to target antigens.

Fc Region

The stem of the antibody structure that binds to immune cells and activates complement proteins.

IgG

The most abundant serum immunoglobulin ($\sim 80\%$) which crosses the placenta and is the primary goal of vaccination.

IgM

The pentameric antibody produced first during a primary immune response, excellent for agglutination and complement activation.

IgA

Immunoglobulin found in mucosal secretions (tears, saliva, breast milk) that prevents microbial adherence to mucosal surfaces.

Neutralisation

The process where antibodies bind toxins or viruses to block their interaction with host cells.

Agglutination

The cross-linking of cells or particles by antibodies to promote easier phagocytosis.

Diapedesis

The process, also known as extravasation, where leukocytes squeeze between endothelial cells to leave capillaries and enter tissues.

Granuloma

An organised, walled-off collection of immune cells formed when pathogens cannot be eliminated by phagocytes during chronic inflammation.