Organisation of the immune system Flashcards
Physical barriers for host defence
Skin - mechanical barrier, acidic environment
Mucous membranes - mucus secretions trap microorganisms, cilia expel microorganisms
Physiological barriers for host defence
Body temperature/fever
Low pH in stomach
Chemical mediators - lysosomes, interferons, complement
Cell types of the immune system
T lymphocytes B lymphocytes Antigen presenting cells Neutrophils Eosinophils Basophils Monocytes/macrophages Mast cells Dendritic cells Natural Killer cells
The two major types of T lymphocyte
CD4+ T lymphocytes
CD8+ T lymphocytes
CD4+ T lymphocyte function
T helper cells
Regulatory T cells
- Secrete cytokines
CD8+ T lymphocyte functions
Cytotoxic T cells
- Lyse infected cells
- Secrete cytokines
Process of antigen recognition in T cells
T cells only recognise processed antigen presented at the surface of another cell upon binding with T cell receptor.
Antigen is presented by an MHC molecule after processing.
B lymphocyte features
- Surface antigen receptor is an Immunoglobulin-like structure
- Express MHC class II which present antigen to T helper cells
- Recognised free, intact antigens in body fluids or on cell surfaces
- Use B cell receptor
Role of antigen presenting cells and examples
- Present processed antigens to T lymphocyte to initiate an adaptive immune response
- Dendritic cells
- B lymphocytes
- Macrophages
Roles of neutrophils
Phagocytosis
Killing of microbes
Roles of eosinophils
Phagocytosis
Granule release
Defence against parasitic infections
Roles of basophils
Granule release
May act as APC
Roles of monocytes/macrophages
Phagocytosis
Killing of pathogens
Cytokine release
Acting as APC
Roles of mast cells
Pro-inflammatory granule release
- Secrete histamine and inflammatory mediators such as cytokines
- Can recognise, phagocytose and kill bacteria
- Activation leads to vasodilation and increased vascular permeability
Roles of dendritic cells
Antigen capture
Antigen presentation
Roles of Natural Killer cells
Lysis of infected cells (NOT pathogens)
What are neutrophil extracellular traps
Activated neutrophils release granule proteins and chromatin to form extracellular fibres which trap and immobilise pathogens
Method used to distinguish white blood cells
Cluster of Differentiation
- Uses CD markers
- System differentiates cells based on which antibodies bind to specific molecules expressed on the cell surface
Definition of a primary lymphoid organ
An organ where lymphocytes develop and congregate
Primary lymphoid organs
- Bone marrow, site of B cell maturation and production of immature T cells
- Thymus, site of T cell maturation
Definition of a secondary lymphoid organ
An organ where lymphocytes interact with antigens and other lymphocytes
Secondary lymphoid organs
The spleen
Lymph nodes
Mucosal associated lymphoid tissues (MALT)
Features of the thymus
- Bi-lobed
- Contains whirls of fibroblasts (Hassall’s corpuscles) site of T cell development
- Size decreases with age (atrophy)
- T-cell variation output decreases with age but total output number remains the same
Features of bone marrow
- Site of haematopoiesis
- Increases white blood cell output during acute-phase response
- 2 types, Red and Yellow
- Red marrow = haematopoietic tissue
- Yellow marrow = fatty tissue
Functions of the lymphatic system
- Drainage system for extracellular fluid
- Provides sites for immune cells to interact
Features of the spleen
- Red and white pulp
- White pulp contains lymphocytes and carries out active immune response
- Red pulp is primarily a filter for the blood, also site of red cell turnover
- PALS periarterial lymphatic sheath is T cell area
- Primary follicles is B cell area
Features of the mucosa associated lymphoid tissues (MALT)
Lymphoid tissues located near sites of likely infection in the mucosa. Provide defence support to epithelia.
Features of Gut Associated Lymphoid Tissue (GALT)
Organised lymphoid tissue - “Peyer’s patches” which contain compartmentalised cells.
Disorganised lymphoid tissue parts are associated with lymph drainage
M-cells sample antigens and transfer them to dendritic cells
Dendritic cells then migrate to Peyer’s patch and present the antigens to lymphocytes for clonal expansion
Problems leading to necessity of lymphocyte recirculation
- Very large number of T cells with different specificities
- Very large number of B cells with different specificities
- May only limited amounts of antigen
- Body needs a way of ensuring lymphocyte meets its specific antigen
Lymphocyte recirculation process
- Naive lymphocytes leave bone marrow/thymus and enter bloodstream
- Travel through blood and either find its complementary antigen or dies
- Recirculates through peripheral lymphoid tissues
Fates of lymphocytes in lymphocyte recirculation
- Due by apoptosis
- Recognise antigen, activation of B cells leading to massive B cell proliferation in secondary lymphoid tissue
Process of extravasation (movement from blood vessel into tissue) of naive T cells into lymph nodes
- Rolling, naive T cells ‘roll’ along epithelium
- Activation, T cells stopped and activated by chemokine at a particular place by selectins
- Arrest and adhesion, integrins increase adhesion of T cell to epithelium leading to arrest of cell
- Transendothelial migration, movement of T cell from epithelial surface into lymph node
