PATH2220 Study Guide - Final Guide: T Helper 17 Cell, Cell-Mediated Immunity, Antigen-Presenting Cell
Immunity
Overview of the Immune System:
• Immunity
o Our ability to protect ourselves from disease
o Recognition and removal of foreign material entering body
o Can be innate or acquired
• Immunology
o Study of cells/organs/molecules responsible for immunity and
how they respond and interact
o Effects and consequences
o Can the response be advantageously increased/reduced
• Immune system
o Cells and molecules responsible for immunity
o Their collective/coordinated response to the introduction of
foreign substances
o Has evolved different methods to deal with the wide variety of
pathogens it has to deal with
o 2 branches
▪ Innate → early into infection
▪ Adaptive → days into response
Innate Immune System:
• Required to cover the time taken for adaptive immunity to be generated
• React quickly and efficiently
• Less specific actions
• Physical components
o Physical barrier between microbes in external environment and
host tissue → skin and mucosal surfaces
o Multiple levels of physical protection
▪ Tight junctions
▪ Keratin
▪ Mucus assisted by cilia and peristalsis
o Epithelial cells also produce antimicrobial chemicals → defensins
→ further impede entry of microbes
o Intraepithelial T cells recognize and respond to a small number of
common microbial structures
• Cellular components
o Macrophages
▪ Large phagocytic tissue cells
▪ Responsible for removal of damaged tissue/cells/bacteria
o Neutrophils
▪ Short lived scavenger blood cells
▪ Contain granules of powerful bactericidal enzymes
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o Dendritic cells
▪ Present antigen to T cells
▪ Initiate adaptive immune responses
o Natural killer (NK) cells
▪ Lymphocyte-like cells
▪ Capable of killing virus infected and tumour cells without
specificity of true lymphocytes
o Mast cells
▪ Found in tissues
▪ Release inflammatory mediators when damaged and under
influence of IgE antibody
• Soluble components
o Several molecules that recognize/respond to microbes
o Promote innate responses
o Exist in blood and ECF → cytokines, chemokines, defensins
o Function
▪ Bind to microbes and enhance phagocytosis
▪ Promote inflammatory responses → bring phagocytes to
site of infection
• What innate immunity recognizes
o Molecular structures that are produced by microbial pathogens
▪ Pathogen associated molecular patterns (PAMPs)
▪ Essential for survival of microbes
▪ Ensures target of immune response cant just be discarded
by microbe
o Endogenous molecules produced by dead/dying cells
▪ Damage associated molecular patterns (DAMPs)
▪ Can be produced as a result of cell damage caused by
infection
▪ Also produced in response to sterile injury to cells
▪ Generally not released by cells dying from apoptosis
o PAMPs and DAMPs are recognized by Pattern Recognition
Receptors (PRR)
▪ Most cell types express PRR and are capable of
participating in innate immune responses
▪ Phagocytes and dendritic cells → widest variety
▪ Expressed on cell surfaces, phagocytic vesicles and in the
cytosol of cells
▪ When receptors bind PAMPs and DAMPs they activate
signal transduction pathways → promote
antimicrobial/proinflammatory functions
• Effector mechanisms
o Inflammation → major reaction to damaged/dead cells and to
accumulation of abnormal substances in cells/tissues
o Anti-viral defense → consist of changes in cells that prevent virus
replication and increase susceptibility
Adaptive Immune System:
• Key cells → B and T lymphocytes
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• Lymphocytes express highly diverse membrane receptors → recognize a
wide variety of foreign substances
• Provides diversity → can respond to a large variety of antigens
• Takes about a week to develop
• Provides long-term memory → ensures a faster, better response when
next encountering the same pathogen
• Types of adaptive immunity
o Humoral immunity
▪ Mediated by secreted antibodies (B cells)
▪ Defense against extracellular microbes
▪ Neutralise infectivity and target microbes for elimination
▪ Antibodies are specialized
o Cell mediated immunity
▪ Mediated by T cells and their products (cytokines)
▪ Defense against intracellular microbes
• T cell activation
o T cells need to recognize foreign pathogen
o Can not directly see microbes → only reognise antigens present
on host cells
o Need a second costimulatory signal
o T cell subsets → all express CD3+
▪ Cytotoxic T cells → directly kill intracellular microbes by
killing infected cells
▪ T helper cells → TH1/TH2/TH17 → CD4+ receptors
• Secrete surface molecules and secrete cytokines
• TH1 cells → activate macrophages to ingest and
destroy internalized microbes
• TH2 cells → stimulate reactions that serve to
eradicate helminthic infections (IL-4, IL-5, IL-13)
• TH17 cells → secrete cytokines that recruit
neutrophils to sites of infection
▪ Regulatory T cell
• B cell activation
o Recognise antigen in its intact, native form
o Do not require antigen to be processed
o Antibody molecules expressed on the surface of the B cell act as
the B cell receptor
o Secondary signals promote increased activation and signaling
o Once activated B cells differentiate into plasma cells that secrete
antibody molecules
o Antibodies exist as different isotypes
o They have different functions at different locations and for
different pathogens
• Effector mechanisms
o Neutralisation
▪ Antibodies prevent binding of microbes to cells and so
inhibit infection of host cell
▪ Antibodies inhibit the spread of microbes from an infected
cell to adjacent uninfected cell
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