Provides defense or immunity against infectious agents ranging from viruses to multicellular parasites
Immune system
Consists of a large, diverse population of leukocytes located within every tissue of the body and lymphoid organs interconnected only by the blood and lymphatic circulation
Two fundamental lines of defense
Innate immunity
Adaptive immunity
Innate immunity
Represents the first line of defense to an intruding pathogen
It is an antigen-independent (non-specific) defense mechanism that is used by the host immediately or within hours of encountering an antigen
Adaptive immunity
Acquired gradually by exposure to microorganisms
More specific
Develops more slowly and is based on antigen presentation to lymphocytes
Responses are aimed at specific microbial invaders and involve production of memory lymphocytes
Antigens
Usually proteins that are recognized by lymphocytes to elicit a specific immune response against them
Antibodies
Immunoglobulins produced by plasmacells after a progenitor B cell is activated by a specific antigen
Major Histocompatibility Complexes (MHC)
Cell surface glycoproteins whose primary function is to present peptide fragments for recognition by the appropriate T cells (lymphocytes)
MHC Class I Molecules are found on surfaces of all nucleated cells and bear fragments of their constituent proteins
MHC Class II Molecules are only found on antigen-presenting cells (APCs)
Two forms of adaptive immune response
Humoral immunity
Cell-mediated immunity
Cell-mediated immunity
Mature T cells lymphocytes, macrophages, and the production of cytokines in response to an antigen are the main drivers
Primary Lymphoid Organs
Bone Marrow for B Lymphocytes
Thymus for T lymphocytes
Secondary Lymphoid Organs
Lymph Nodes
MALT
Spleen
Thymus
Bilobed organ in the mediastinum that is most active and prominent before puberty and undergoes involution with less activity in the adult
Found in the midline of the thoracic cavity, that is surrounded by the left and right pleural sacs
Primary organ where T Cells are produced
Originates from endoderm
Thymic Medulla
Contains fewer and larger, more mature lymphocytes
Cytoreticulum, supports T lymphocytes, dendritic cells and macrophages; expresses many specialized proteins specific to cells of other organs
Secondary Layer- serves as boundary between cortex and medulla
Hassall Corpuscles (Aggregates of TEC)
Mucosa-Associated Lymphoid Tissue (MALT)
Found in the mucosa of most tracts but is concentrated in the palatine, lingual and pharyngeal tonsils, Peyer patches, and the appendix
Collectively the MALT is one of the largest lymphoid organs, containing up to 70% of all the body's immune cells. Most of the lymphocytes here are B cells; among T cells, CD4+ helper T cells predominate
Lymph Nodes
Bean-shaped, encapsulated structures, generally only 10 mm by 2.5 cm in size, distributed throughout the body along the lymphatic vessels
Filters lymph
Site for B-cell activation and differentiation
Compartments of Lymph Nodes
Outer Cortex - point of entry of lymphocytes to the entire Lymph Node
Paracortex - High Endothelial Venules (HEVs) portal of entry of lymphocytes
Inner Medulla - Medullary Cords, Medullary Sinuses, Hilum - Where blood vessels and nerve(s) enter
Spleen
Only lymphoid organ involved in filtration of blood, making it an important organ in defense against blood-borne antigens
It is also the main site of old erythrocyte destruction
Without a cortex/medulla structure instead two intermingled but functionally different regions: white pulp and red pulp
Parts of the Red Pulp
Splenic Cords (Cords of Billroth) - contains macrophages, reticular cells and fibers, other leukocytes
Splenic Sinusoids - lined by unusual endothelial cells called stave cells that are elongated and aligned parallel to the blood flow
Innate immunity
Involves leukocytes (mainly granulocytes), and proteins such as defensins, complement, lysozyme, and interferons; cytokines
Hydrochloric acid (HCI)
Antimicrobial chemical
Defensins
Short cationic polypeptides produced by neutrophils and various epithelial cells that kill bacteria by disrupting the cell walls
Lysozyme
An enzyme made by neutrophils and cells of epithelial barriers, which hydrolyzes bacterial cell wall components, killing those cells
Complement
A system of proteins in blood plasma, mucus, and macrophages that react with bacterial surface components to aid removal of bacteria
Interferons
Paracrine factors from leukocytes and virus-infected cells that signal NK cells to kill such cells and adjacent cells to resist viral infection
Forms of adaptive immune response
Humoral immunity
Cell-mediated immunity
Humoral immunity
Antibodies are primarily responsible for the response
B cell lymphocytes, a type of immune cell that makes antibodies after detecting a specific antigen, are principally responsible for this method
ThymicCortex
Darkly basophilic
Contains an extensive population of T lymphoblasts (or thymocytes) located among numerous macrophages and associated with the unique thymic epithelial cells (TECS) that have certain features of both epithelial and reticular cells
Squamous Cells
Blood-thymus barrier
StellateEpithelialCells
Cytoreticulum, secrete numerous cytokines for T-cell development
SquamousCorticalCells
Corticomedullary barrier forms sheet like structure
Thymic Medulla
Contains fewer and larger, more mature lymphocytes
Cytoreticulum
Supports T lymphocytes, dendritic cells and macrophages; expresses many specialized proteins specific to cells of other organs
Secondary Layer
Serves as boundary between cortex and medulla
HassallCorpuscles
Aggregates of TEC
ThymicCortex
Darkly basophilic
Contains an extensive population of T lymphoblasts (or thymocytes) located among numerous macrophages and associated with the unique thymic epithelial cells (TECS) that have certain features of both epithelial and reticular cells
Thymic Cortex Cell Types
Squamous Cells - Blood-thymus barrier
Stellate Epithelial Cells - Cytoreticulum, secrete numerous cytokines for T-cell development
Squamous Cortical Cells - Corticomedullary barrier forms sheet like structure
Thymic Medulla
Contains fewer and larger, more mature lymphocytes
Thymic Medulla Components
Cytoreticulum, supports T lymphocytes, dendritic cells and macrophages; expresses many specialized proteins specific to cells of other organs
Secondary Layer - serves as boundary between cortex and medulla