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Module 15 in the Syllabus

The Lymphatic System and Immunity Images
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Overview of The Lymphatic System and Immunity
The immune system is composed of the responses by which the body neutralizes foreign agents, whether living or nonliving, as well protects against its own cells which may be cancerous. These responses fall into two general categories. General protective mechanisms which function without prior exposure to harmful agents are classified as nonspecific defense mechanisms. Acquired immunity consists of specific immune responses directed against specific agents. Acquired immunity depends on the lymphatic system, a network of lymphatic vessels that carry a clear fluid called lymph containing lymphocytes and other white blood cells.

The Lymphatic System and Immunity on the MCAT

Conceptual Vocabulary
Immune systemThe immune system is a collection of mechanisms within the body that protects against disease by identifying and killing pathogens and tumor cells.
Lymphatic systemThe lymphatic system is a complex network of lymphoid organs, lymph nodes, lymph ducts, tissues, lymph capillaries and lymph vessels that produce and transport lymph fluid from tissues to the circulatory system.
PathogenA pathogen or infectious agent is a biological agent that causes disease or illness to its host.
TumorA tumor is an abnormal growth or mass of tissue.
InfectionAn infection is the detrimental colonization of a host organism by a foreign species.
White blood cellWhite blood cells or leukocytes are cells of the immune system which defend the body against both infectious disease and foreign materials.
AntigenAn antigen or immunogen is a molecule that stimulates an immune response.
AntibodiesAntibodies or immunoglobulins are proteins used by the immune system to identify and neutralize foreign objects, such as bacteria and viruses.
Innate immune systemThe innate immune system comprises the cells and mechanisms that defend the host from infection by other organisms, in a non-specific manner.
InflammationInflammation is the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants.
Complement systemThe complement system is a biochemical cascade which helps clear pathogens from an organism consisting of a number of small proteins found in the blood, normally circulating as inactive zymogens.
Adaptive immune systemThe adaptive immune response provides the vertebrate immune system with the ability to recognize and remember specific pathogens.
T cellT cells are lymphocytes which play a central role in cell-mediated immunity which can be distinguished from other lymphocyte types, such as B cells and NK cells by the presence of a special receptor on their cell surface.
Cell-mediated immunityNot involving antibodies, cell-mediated immunity is an immune response involving the activation of macrophages, natural killer cells, cytotoxic T-lymphocytes, and the release of various cytokines in response to an antigen.
Natural killer cellNatural killer cells are a form of cytotoxic lymphocyte which constitute a major component of the innate immune system.
Humoral immunityThe humoral immune response is the aspect of immunity that is mediated by antibodies, produced by the cells of the B lymphocyte lineage.
B cellB cells are lymphocytes that play a large role in the humoral immune response.
Plasma cellPlasma cells are cells of the immune system that secrete large amounts of antibodies.
Memory B cellMemory B cells are a B cell sub-type that are formed following primary infection. They remember the same pathogen for faster antibody production in future infections
AllergyAllergy is an abnormal reaction to a substance foreign to the body that is acquired, predictable and rapid.
AllergenAn allergen is a nonparasitic antigen capable of stimulating a type I hypersensitivity reaction in atopic individuals.
Lymph nodeLymph nodes are components of the lymphatic system that contain white blood cells and act as filters or traps for foreign particles.
Lymph vesselLymph vessels are thin walled, valved structures that carry lymph.
Neutrophil granulocyteNeutrophils are the most abundant type of white blood cells, phagocytes that quickly congregate at a focus of infection, attracted by cytokines expressed by activated endothelium, mast cells and macrophages.
CytokineCytokines are a family of small, signaling proteins and glycoproteins particularly important in both innate and adaptive immune responses.
PhagocyteA phagocyte is a cell that ingests and destroys foreign matter such as microorganisms or debris.
MacrophageMonocytes and macrophages are phagocytes, acting in both innate immunity as well as cell-mediated immunity of vertebrate animals.
Eosinophil granulocyteEosinophil granulocytes are white blood cells of the immune system that are responsible for combating infection by parasites in vertebrates
Antigen presentationAntigen presentation is a process in the body's immune system by which macrophages, dendritic cells and other cell types capture antigens and then show them for recognition by T-cells.
Major histocompatibility complexThe major histocompatibility complex (MHC) is a large gene family found in most vertebrates, the most gene-dense region of the mammalian genome, which plays an important role in the immune system, autoimmunity, and reproductive success.
T cell receptorThe T cell receptor is a molecule found on the surface of T lymphocytes that is generally responsible for recognizing antigens bound to major histocompatibility complex molecules.
T helper cellWith no cytotoxic or phagocytic activity, T helper cells are a sub-group of lymphocytes playing an important role in establishing and maximizing the capabilities of the immune system.
Cytotoxic T cellA cytotoxic T cell belongs to a sub-group of T lymphocytes which are capable of inducing the death of infected somatic or tumor cells.
Regulatory T cellRegulatory T cells, sometimes known as suppressor T cells, act to suppress activation of the immune system and thereby maintain immune system homeostasis and tolerance to self-antigens.
EpitopeAn epitope is the part of a macromolecule that is recognized by the immune system, specifically by antibodies, B cells, or T cells.
Thoracic ductThe thoracic duct is the largest lymphatic vessel in the body, collecting most of the lymph in the body and draining into the systemic circulation at the left subclavian vein
SpleenThe spleen is an organ located in the abdomen of the human body, where it carries out immune functions as well as functions concerned with the destruction of old red blood cells.
Autocrine signallingAutocrine signalling is a form of hormonal signalling in which a cell secretes a hormone, or chemical messenger, that binds to receptors on the same cell.
Paracrine signallingParacrine signaling is a form of cell signaling in which the target cell is close to the signal releasing cell.
HistamineHistamine is a biogenic amine involved in local immune responses as well as regulating physiological function in the gut and acting as a neurotransmitter.
Mast cellA mast cell is a resident cell of several types of tissues and contains many granules rich in histamine and heparin.
MonocyteA monocyte is a leukocyte that protects against blood-borne pathogens and moves quickly to sites of infection in the tissues. These cells are identified in stained smears by their large bilobate nucleus.
MHC class IA primary classes of major histocompatibility complex molecules, MHC class I molecules are found on almost every nucleated cell of the body.
MHC class IIMHC Class II molecules are found only on a few specialized cell types, including macrophages, dendritic cells and B cells, all of which are antigen-presenting cells.
Antigen processingAntigen processing is a biological process that prepares antigens for presentation to special cells of the immune system called T lymphocytes.
Immunoglobulin EImmunoglobulin E, a type of antibody, plays an important role in allergy and has also been implicated in immune system responses to most parasitic worms.
Immunoglobulin GBuilt of two heavy chains and two light chains, immunoglobulin G is the most abundant immunoglobulin, constituting 75% of serum immunoglobulins in humans
Monoclonal antibodiesMonoclonal antibodies are antibodies that are identical because they are produced by one type of immune cell that are all clones of a single parent cell.
Reticuloendothelial systemThe reticuloendothelial system consists of the phagocytic cells located in reticular connective tissue, primarily monocytes and macrophages.
ThymusThe thymus is an organ located in the upper anterior portion of the chest cavity just behind the sternum. It is of central importance in the maturation of T cells.
InterleukinInterleukins are a group of cytokines that were first seen to be expressed by leukocytes as a means of communication.
Basophil granulocyteThe least common of the granulocytes, basophils degranulate to release histamine, the proteoglycans heparin and chondroitin, and proteolytic enzymes.
DegranulationDegranulation is a cellular process that releases antimicrobial cytotoxic molecules from secretory vesicles called granules found inside some cells.
GranuleA granule can be any structure barely visible by light microscopy. The term is most often used to describe a secretory vesicle.
GranulocyteGranulocytes are a category of white blood cells characterised by the presence of granules in their cytoplasm.
Dendritic cellDendritic cells are immune cells with branched projections whose main function is to process antigen material and present it on the surface to other cells of the immune system.
Antigen-presenting cellAn antigen-presenting cell is a cell that displays foreign immunogen material complexed with MHC on its surface. T-cells may recognize this complex using their T-cell receptor.
Heavy chainA heavy chain is the large polypeptide subunit of an antibody.
Light chainA light chain is the small polypeptide subunit of an antibody.
Polyclonal antibodyPolyclonal antibodies are antibodies that are derived from different B-cell lines.
AutoantibodyAn autoantibody is an antibody manufactured by the immune system that is directed against one or more of the individual's own proteins.
Immune toleranceImmune tolerance is the process by which the immune system does not attack an antigen.
Central toleranceCentral tolerance is the mechanism by which newly developing T cells and B cells are rendered non-reactive to self.
Reticular connective tissueReticular connective tissue is a type of loose irregular connective tissue with a network of fibers that form a soft skeleton to support lymphoid organs such as lymph nodes, red bone marrow, thymus, and spleen.
LymphokineLymphokines are a subset of cytokines that are produced by a type of immune cell known as a lymphocyte.
Immunoglobulin superfamilyThe immunoglobulin superfamily is a large group of cell surface and soluble proteins that are involved in the recognition, binding, or adhesion processes of cells.
OpsoninAn opsonin is any molecule that acts as a binding enhancer for the process of phagocytosis, for example, by coating the negatively-charged molecules on the membrane.
ChemokineChemokines are a family of small cytokines characterized by the presence of four cysteine residues in conserved locations that are key to forming their 3-dimensional shape.
InterferonInterferons (IFNs) are natural proteins produced by the cells of the immune system of most vertebrates in response to challenges by foreign agents such as viruses, bacteria, parasites and tumor cells.
Respiratory burstRespiratory burst is the rapid release of reactive oxygen species from different types of cells.
Advanced terms that may appear in context in MCAT passages
Specific granulesSpecific granules are a secretory vesicles found exclusively in cells of the immune system called granulocytes. They are also known as secondary granules.
Complement membrane attack complexThe membrane attack complex is typically formed on the surface of intruding pathogenic bacterial cells as a result of the activation of the complement system.
HistiocyteA histiocyte is a monocyte derived tissue macrophage cell.
MicrogliaMicroglia are a type of glial cell that act as the immune cells of the central nervous system, acting as phagocytes, cleaning up CNS debris.
Gamma/delta T cellsGamma/delta T cells represent a small subset of T cells that possess a distinct type of T cell receptor (TCR) on their surface.
Complementarity determining regionA [complementarity] determining region is a short amino acid sequence found in the variable domains of antigen receptor proteins that complements an antigen.
VJ recombinationV(D)J recombination is a mechanism that occurs in vertebrates to randomly selects and assembles segments of genes encoding specific proteins with important roles in the immune system.
Immunoglobulin AImmunoglobulin A is an antibody which is the main immunoglobulin found in mucous secretions.
Immunoglobulin MImmunoglobulin M is a basic antibody that is present on B cells. It is the primary antibody against A and B antigens on red blood cells.
Immunoglobulin allotypeAn immunoglobulin allotype is the allele of the antibody chains found in the individual.
Somatic hypermutationSomatic hypermutation is a mechanism inside cells that diversifies the receptors that the immune system uses to recognize antigens.
Peyer's patchesPeyer's patches are secondary lymphoid organs which are aggregations of lymphoid tissue usually found in the lowest portion of the small intestine in humans
Cytokine stormA cytokine storm is a potentially fatal immune reaction consisting of a positive feedback loop between cytokines and immune cells.
Leukocyte extravasationLeukocyte extravasation refers to the movement of leukocytes from the circulatory system into the interstitial fluid, towards the site of tissue damage or infection.
Major basic proteinMajor basic protein is a 117-residue protein that predominates in eosinophil granulesa which is a potent enzyme against helminths and is toxic towards bacteria and mammalian cells in vitro.
EicosanoidEicosanoids are signaling molecules derived from omega-3 or omega-6 fats. They exert complex control over many bodily systems, especially in inflammation, immunity and as messengers in the central nervous system.
ProstaglandinA prostaglandin is any member of a group of lipid compounds containing 20 carbon atoms, including a 5-carbon ring, which are derived enzymatically from fatty acids and have important functions in the animal body.
Kupffer cellKupffer cells are specialized macrophages located in the liver that form part of the reticuloendothelial system whose primary function is to recycle old red blood cells that no longer are functional.
Classical complement pathwayThe classical pathway of activation of the complement system is a group of blood proteins that mediate the specific antibody response, triggered by the binding of a specific part of the antibody molecule to the C1 component.
Dust cellA dust cell, or alveolar macrophage, is a type of macrophage found in the pulmonary alveolus which reside on respiratory surfaces and clean off particles such as dust or microorganisms.
GranzymeGranzymes are exogenous serine proteases released by cytoplasmic granules within cytotoxic T cells and natural killer cells.
PerforinPerforin is a cytolytic protein found in the granules of CD8 T-cells and NK cells. Upon degranulation, perforin inserts itself into the target cell's plasma membrane, forming a pore.
LeukotrieneLeukotrienes are naturally produced eicosanoid lipid mediators, which may be responsible for a number of the effects of asthma and allergies.
LipoxinLipoxins are short lived endogenously produced eicosanoids whose appearance in inflammation signals the resolution of inflammation.
LactoferrinFound in milk and many mucosal secretions, lactoferrin is a globular multifunctional protein with antimicrobial activity which is part of the innate defense.
Cross-presentationThe term cross-presentation denotes the ability of certain antigen-presenting cells to take up, process and present extracellular antigens to cytotoxic T cells by means of MHC class I molecules.
IsotypeThe immunoglobulin isotype refers to the type of chain of an antibody.
CD8CD8 is a transmembrane glycoprotein which serves as a co-receptor for the T cell receptor.
IdiotypeAn idiotype is a shared characteristic between a group of immunoglobulin or T cell receptor molecules based upon the antigen binding specificity and therefore structure of their variable region.
B-1 cellB-1 cells are B cells that express CD5, which can bind to another B cell surface protein, CD72.
Immunoglobulin class switchingImmunoglobulin class switching is a biological mechanism that changes an antibody from one class to another.
ThymocyteThymocytes are T cell precursors which develop in the thymus.
Glycoprotein 130Glycoprotein 130 is a transmembrane protein which is the founding member of the class of tall cytokine receptors.
Interleukin-1Interleukin-1 is one of the first cytokines ever described, discovered as a factor that could induce fever, control lymphocytes, increase the number of bone marrow cells and cause degeneration of bone joints.
Granulocyte macrophage colony-stimulating factorGranulocyte-macrophage colony-stimulating factor (GM-CSF) is a cytokine that functions as a white blood cell growth factor. It is secreted by macrophages, T cells, mast cells, endothelial cells and fibroblasts.
L-selectinL-selectin, also known as CD62L, is a cell adhesion molecule found on leukocytes. It belongs to the selectin family of proteins, which recognise sialylated carbohydrate groups.
Interferon-gammaInterferon-gamma is a dimerized soluble cytokine that is the only member of the type II class of interferons. This interferon was originally called macrophage-activating factor.
MonokineA monokine is a type of cytokine produced primarily by monocytes and macrophages.
RANTESRegulated upon Activation, Normal T-cell Expressed, and Secreted, or RANTES, is a protein classified as a chemotactic cytokine or chemokine.
CathelicidinMembers of the cathelicidin family of antimicrobial polypeptides are characterized by a highly conserved region called the cathelin domain and also a highly variable domain.
DefensinConsisting of 15-20 amino acids including six to eight conserved cysteine residues, defensins are small cysteine-rich cationic proteins which are active against bacteria, fungi and enveloped viruses.
Mannan-binding lectin pathwayThe Mannan-binding lectin pathway is homologous to the classical complement pathway of activation of the complement system, using a protein similar to C1q of the classical complement pathway.
Alternative complement pathwayNot requiring a specific antibody to commence, the alternative pathway of the complement system is a humoral component of the immune system's natural defence against infections.
CD31CD31 is a cluster of differentiation molecule. It is also called PECAM-1 for platelet endothelial cell adhesion molecule, playing a key role in removing aged neutrophils from the body.
SuperantigenProduced by certain microbes as a defense mechanism, superantigens are secreted proteins that exhibit highly potent lymphocyte-transforming activity directed towards T lymphocytes.
ParacortexParacortex is the portion of a lymph node immediately surrounding the cortex. Unlike the cortex, which has mostly B cells, this structure has mostly T cells.
Cords of BillrothThe Cords of Billroth are found in the red pulp of the spleen between the sinusoids, consisting of fibrils and connective tissue cells with a large population of monocytes and macrophages.

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