28 Questions
What is the approximate percentage of NK cells in the blood and secondary lymphoid organs?
5% to 20%
Which of the following is true about NK cells' antigen receptors?
They do not express immunoglobulins or TCRs
What activates macrophages to kill phagocytosed microbes?
IL-12
What is the function of NK cells in response to infected cells?
Induce apoptosis in infected cells
Which of the following is a function of cytokines in innate immunity?
Recruiting blood neutrophils and monocytes
Which complement activation product leads to opsonization and phagocytosis through its deposition on microbes?
$C3b$
Which cytokine is induced by bacterial PAMPs and can lead to septic shock characterized by low blood pressure and disseminated intravascular coagulation?
$TNF$
Which type of IFNs, produced in viral infections, inhibit viral replication and prevent spread of the infection to uninfected cells?
Type I IFNs
What is the function of C5b in complement activation?
$Formation$ of membrane attack complex (MAC) causing lysis of the microbe
Which cells secrete cytokines in innate immunity in response to microbes or other cytokines?
$Dendritic$ cells
What are the effector functions of products from alternative, classical, and lectin pathways of complement activation?
Both inducing inflammation and opsonization/phagocytosis
Which cytokine activates NK cells and macrophages?
IL-12
What is the main function of C3a released during complement activation?
Inducing inflammation
Which cells produce type I IFNs in viral infections?
Dendritic cells
What is the potent stimuli for cytokine secretion in innate immunity?
Microbial nucleic acids
Which complement pathway is triggered by antibodies binding to microbes?
Classical pathway
Which plasma protein recognizes microbial carbohydrates and activates the complement cascade?
Plasma MBL
What is the central component of all three complement pathways?
C3
Which immune cells compensate for defective CTL response in the host-microbe struggle?
NK cells
What are the main functions of the complement system in host defense?
Opsonization and phagocytosis, inflammation, and cell lysis
Which pathway is a component of innate immunity, triggered when complement proteins are activated on microbial surfaces without control due to the absence of complement regulatory proteins on microbes?
Alternative pathway
Which cytokines are known to activate NK cells?
IL-15, type I IFNs, and IL-12
What is the function of inhibitory receptors on NK cells?
Block signaling by activating receptors and are specific for self class I major histocompatibility complex (MHC) molecules
What is the principal function of NK cells in antibody-dependent cellular cytotoxicity (ADCC)?
Mediating the killing of antibody-coated target cells
What determines NK cell responses?
A balance between engaging activating and inhibitory receptors
How do activated macrophages contribute to NK cell function?
Release IL-12, which produces IFN-gamma from NK cells, leading to the killing of phagocytosed microbes
What is the role of activating receptors on NK cells?
Enable NK cells to eliminate cells infected with intracellular microbes, irreparably injured cells, and tumor cells
How do many viruses evade killing by NK cells?
They downregulate ligands for activating receptors
Study Notes
Natural Killer (NK) Cell Activation and Function
- NK cells bind to virus-infected cells and kill them
- Activated macrophages release IL-12, which produces IFN-gamma from NK cells, leading to the killing of phagocytosed microbes
- Cytokines secreted by macrophages and dendritic cells enhance NK cells' ability to protect against infections
- IL-15, type I IFNs, and IL-12 are NK cell-activating cytokines
- NK cells and macrophages function cooperatively to eliminate intracellular microbes
- NK cell responses are determined by a balance between engaging activating and inhibitory receptors
- Activating receptors enable NK cells to eliminate cells infected with intracellular microbes, irreparably injured cells, and tumor cells
- NK cells are the principal mediators of antibody-dependent cellular cytotoxicity (ADCC)
- Activating receptors on NK cells have signaling subunits containing immunoreceptor tyrosine-based activation motifs (ITAMs)
- Inhibitory receptors of NK cells block signaling by activating receptors and are specific for self class I major histocompatibility complex (MHC) molecules
- Many viruses have developed mechanisms to block expression of class I molecules in infected cells to evade killing by NK cells
- The text includes a figure depicting the engagement of inhibitory and activating receptors of NK cells with healthy and infected cells.
Test your knowledge about complement activation and the peptide by-products C3a and C5a. Explore the functions and regulation of the complement system as well as the effector functions of products obtained from different pathways.
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