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Microbiology 204: Cellular and Molecular Immunology
Class meets MWF 11-12:30
Lectures are open to auditors
Discussions are restricted to those enrolled in class (or by permission)
Textbook recommended: Janeway et al Immunobiology or Abbas and Lichtman Cellular and Molecular Immunology
Microbiology 204: Cellular and Molecular Immunology
Grades: 2/3 take-home final and 1/3 participation in discussions
My office hours: Mondays 4-5PM HSE1001F ([email protected])
The central questions
• How does the immune system respond to different infections?– Microbes are recognized by two mechanisms, evolved
broad recognition mechanisms (innate immunity), and by highly specific lymphocyte antibodies and T cell receptors (adaptive immunity)
– Different types of microbes are eliminated by different effector mechanisms, which are designed to best combat each type of microbe
• Why does the immune system not respond to self antigens?
• What are the pathogenic mechanisms and clinico-pathologic consequences of abnormalities in the immune system?
Cells of the immune system
• Lymphocytes– Mediators of adaptive immune responses; only
cells with specific receptors for antigens• Antigen-presenting cells (APCs)– Specialized to capture, concentrate, and display
antigens for recognition by lymphocytes– Dendritic cells; macrophages, B cells; follicular
dendritic cells– Different APCs serve different roles in adaptive
immune responses• Effector cells– Function to eliminate microbes; include
lymphocytes, granulocytes (neutrophils, eosinophils), macrophages
Innate Immune component Recognition Property Function
Toll-like receptors Cell wall components;Nucleic acids
Trigger inflammationPromote adaptive response
Collectins Carbohydrate structures Agglutination, phagocytosis,complement activation
Alternative pathway ofcomplement
Membranes lacking proteinsthat block it
Damage to cells; promotephagocytosis
Apoptotic sensors withincells (p53, etc.)
Stress within cell,unscheduled DNA replic.Presence of dsRNA
Killing of virus-infected cell
Examples of Innate Immune Recognition
Principal mechanisms of defense against microbes
Antibodies Phagocytes T cells (CTLs) (may work with antibodies, T cells)
All microbes
All microbes
Intracellular microbes, esp.
viruses
Great variability of antigen recognition is created by
combination of gene segments during lymphocyte development
CD Nomenclature
• Structurally defined leukocyte surface molecule that is expressed on cells of a particular lineage (“differentiation”) and recognized by a group (“cluster”) of monoclonal antibodies is called a member of a cluster of differentiation (CD)
• CD molecules (CD antigens, CD markers) are:• Identified by numbers• Used to classify leukocytes into functionally
distinct subpopulations, e.g. helper T cells are CD4+CD8-, CTLs are CD8+CD4-
• Often involved in leukocyte functions• Antibodies against various CD molecules are used to:
• Identify and isolate leukocyte subpopulations• Study functions of leukocytes• Eliminate particular cell populations
Two types of MHC
Coordination of properties with functions of two types of T cells: source of peptide and cells expressing
Two types of T cells: coordination of function with properties of antigen-
presentation
CD4 T cells .Help other immune cellsRecognize peptide + MHC IIMHC II is expressed primarily on immune cellsPeptides are from endocytosed antigen
CD8 T cells .Kill virus-infected cellsRecognize peptide + MHC IMHC I is expressed on all nucleated cellsPeptides are from cytosolic antigen
Generation of lymphocytes of many specificies
Clonal deletion to remove self-reactive lymphocytes
Clonal selection to expand pathogen-reactive lymphocytes during an immune response
Anatomy of a lymph node
Naïve lymphocytes circulate between blood and lymphoid tissues; antigen in tissue arrives at draining lymph node via lymph flow and being carried by dendritic cells
Applies to B cells and T cellsFor T cells: costimulatory molecules include B7-1 and B7-2 on dendritic cells
Mechanism for directing the immune response against microbes and not against self, food, etc.
Stages of lymphocyte activation
• Naïve lymphocytes– Mature lymphocytes that have not previously encountered
antigen; function -- antigen recognition– Preferential migration to peripheral lymphoid organs (lymph
nodes), the sites where immune responses start
• Effector lymphocytes– Activated lymphocytes capable of performing the functions
required to eliminate microbes (‘effector functions”)– Effector T lymphocytes: cytokine secretion (helper cells),
killing of infected cells (CTLs)– B lymphocytes: antibody-secreting cells (e.g. plasma cells)
• Memory lymphocytes– Long-lived, functionally silent cells; mount rapid responses
to antigen challenge (recall, or secondary, responses)
Immune responses often can be characterized as type 1 or type 2
• Type 1 immune responses: Killing microbes– Pro-inflammatory;
neutrophils and macrophages
– Antibody classes involved in phagocytosis and complement activ.
– Macrophage activation
• Type 2 immune responses: Defense at epithelium– Allergic
inflammation: eosinophils, basophils
– Antibody classes: IgE and IgG1 (mast cell activation)
– Expulsion type reactions (diarrhea, coughing, sneezing, etc.).
Congenital immunodeficiency diseases are often caused by blocks at different stages of lymphocyte maturation
LYMPHOCYTE DEVELOPMENT