CPAT3201 - Lectures 7+8

What is the purpose of the immune response?

to recognize 'self', eliminate pathogens, clear debris, remove 'sick' or cancerous cells

What are the common sites of pathogen entry into the body?

mouth, cornea/conjunctiva, epidermis, respiratory tract, GI tract, GU tract, anus

What are the main types of PMNs (POLYMORPHONUCLEAR) and their roles?

neutrophils (kill bacteria), eosinophils (parasites), basophils (early allergy, become mast cells), monocytes (bacteria, debris, inflammation)

Which immune cells are pre-primed to respond immediately to pathogens?

neutrophils, eosinophils, basophils, NK cells

What are the three types of immune barriers?

physical (skin, mucosa), chemical (low pH, antimicrobials), physiological (coughing, sneezing, diarrhea)

What is the function of probiotic flora in immunity?

compete for nutrients, block adhesion sites, stimulate immunity

What are the two phagocytic killing mechanisms in innate immunity?

oxygen-independent (lysosomal enzymes), and oxygen-dependent (oxidative burst via ROS/RNS).

What enzymes are used in oxygen-independent phagocytosis?

collagenase, gelatinase, phospholipases, serine proteases

What is the key enzyme in neutrophils that gives pus a green color?

myeloperoxidase (MPO), which forms hypochlorous acid from H₂O₂

What are the three cytolytic mechanisms used by immune cells?

1. Non-secretory (Fas/FasL), 2. Secretory (perforin + granzyme), 3. ADCC (antibody-dependent cytotoxicity)

What are cytokines and which cells secrete them?

small proteins acting on immune cells, secreted mainly by lymphocytes, macrophages, DCs, epithelial and connective tissue cells

What are chemokines and their role?

chemotactic cytokines that guide leukocyte movement, increase integrin affinity, and maintain tissue architecture

What are the three complement activation pathways?

classical (C1 + IgG/IgM), alternative (microbial surface), lectin (binds microbial carbs, activates C1)

What are the main functions of the complement system?

opsonization, cell lysis via MAC, and leukocyte recruitment (all mediated via C3b)

What are the differences between IgM and IgG in terms of binding?

IgM: low affinity, high avidity (pentamer); IgG: high affinity, lower avidity (monomer)

What are PRRs and what do they detect?

Pattern Recognition Receptors, they detect PAMPs and DAMPs on pathogens or damaged cells

What are examples of PAMPs?

[pathogen-associated molecular patterns]LPS, mannose, dsRNA, unmethylated CpG DNA

What are DAMPs and when are they released?

[damage-associated molecular patterns]=endogenous molecules released by damaged or dying cells, trigger sterile inflammation

Where are TLRs located and what do they recognize?

cell surface (e.g., TLR4 → LPS) and endosomes (e.g., TLR3 → viral dsRNA); they recognize microbial and danger signals

What are the key effects of TLR activation?

cytokine/chemokine production, activation of killing mechanisms, DC activation

What is the function of dendritic cells in immunity?

immature DCs capture antigens; mature DCs present them to T cells in lymph nodes, initiating adaptive immunity

What is the goal of the innate immune response?

to rapidly reduce pathogen load and prevent the need for adaptive immunity

What are the roles of antigen-presenting cells (APCs)?

trap/process antigens, migrate to lymph nodes, express high MHC I/II, secrete cytokines (e.g., IL-12)

What is the function of plasmacytoid dendritic cells?

secrete large amounts of IFN-gamma in response to viral infection

What guides immune cells to infection sites?

chemokines and cytokines creating chemotactic gradients and endothelial adhesion molecule expression

What is the purpose of lymph node shutdown during immune activation?

traps lymphocytes to allow clonal expansion and antigen-specific response before release

What are the 4 phases of the adaptive immune response?

recognition, activation, execution (effector functions), and control (down-regulation)

What are key features of adaptive immunity?

specificity, memory, self-limitation, MHC antigen presentation

Which MHC class presents to CD4+ and CD8+ T cells?

MHC I → CD8+, MHC II → CD4+

What are the three signals required for full T cell activation?

1. TCR binds antigen/MHC, 2. CD4/CD8 binds MHC, 3. CD28 binds CD80/86.

What cytokine promotes T cell proliferation after activation?

IL-2

What limits T cell expansion after activation?

CTLA-4 (CD152) binds CD80/86 to inhibit further stimulation

Which cytokines direct CD4+ T cell differentiation?

IL-12 → Th1, IL-4 → Th2, IL-6/IL-23 → Th17

What causes decline of T cell response post-infection?

reduced antigen, IL-2, and co-stimulation → leads to apoptosis of most effector T cells

Why do memory T cells survive longer than effector T cells?

they retain anti-apoptotic proteins (e.g., Bcl-2) and don't require IL-2 for survival

How do B cells recognize antigens differently from T cells?

BCRs recognize unprocessed, native antigens (not peptide-MHC complexes)

What is affinity maturation in B cells?

repeated antigen exposure decreases antibody dissociation constant (Kd), increasing specificity

What stimulates antibody class switching in B cells?

T-dependent antigens + cytokines | IFN-γ → IgG2a, IL-4 → IgG1/IgE, TGF-β → IgA

Do T-independent antigens cause antibody class switching?

no, they mainly stimulate IgM production only