The two chains of the T cell receptor

The two chains of the T cell receptor.

The immune system is composed of the responses by which the body neutralizes foreign agents, whether living or nonliving, as well as 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.

Mastery of immune system concepts is a major challenge for MCAT review. Study of this material should begin as early as possible in MCAT review. There is a large body of factual information and some aspects are conceptually difficult. You need multiple study cycles over the course of MCAT review to get where you need to be.

WikiPremed Resources

The Lymphatic System and Immunity Images
Image gallery for study with links to larger teaching JPEGs for classroom presentation

Question Drill for The Lymphatic System and Immunity
Conceptual Vocabulary Self-Test

Basic Terms Crossword Puzzle

Basic Puzzle Solution

Learning Goals

Proficiency 

Understand the distinctions between general & specific, innate & aquired, and cell-mediated & humoral as immune system concepts.

Understand the definition of an antigen.

Be able to name the various categories and individual types of leukocytes from memory.

Be prepared to describe the anatomy and mechanics of circulation within the lymphatic system.

Grasp the roles in the lymphatic system of the bone marrow, thymus, lymph nodes, tonsils, spleen, Peyer's patches and the appendix.

Be familiar with the roles of skin, mucus, sebum, tears and various enzymes such as lysozyme and Rnase in nonspecific/innate defense.

Be able to describe the activities of natural killer cells.

Be able to describe the activities of neutrophils, basophils and eosinophils.

Be familiar with the reticuloendothelial system and the phagocytic nature and mechanisms of antigen presentation by macrophages and dendritic cells.

Understand the mechanisms of the CD (cluster of differentiation) system, MHC (major histocompatibility complex), B cell receptor, and T cell receptor in antigen presentation and recognition.

Be able to describe the model presented in clonal selection theory for how certain B and T lymphocytes are selected for proliferation.

In cell mediated immunity, be able to describe and distinguish the activities of cytotoxic T cells, helper T cells, suppressor T cells, and memory T cells.

In humoral immunity, understand the purposes of helper T cells, plasma B cells and memory B cells.

Be familiar with the structure and functions of the various classes of antibodies.

Understand the various modes of activity of the complement system including the classical pathway, membrane attack complex, and alternative pathway.

Know the features and steps of inflammation and understand how mast cells function in both innate and specific immunity.

Suggested Assignments

Review the terminology for the immune system using the question server. Complete the fundamental terms crossword puzzle. Here is the solution to the puzzle.

Complete the advanced crossword puzzle for the immune system. Here is the solution to the puzzle.

Read pp. 126-137 in ExamKrackers Biology II. Perform practice items 89-96 on pg. 138.

Review the web resources for the lymphastic system and immunity.

Conceptual Vocabulary for The Lymphatic System and Immunity

Immune system
The immune system is a collection of mechanisms within the body that protects against disease by identifying and killing pathogens and tumor cells.
Lymphatic system
The 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.
Pathogen
A pathogen or infectious agent is a biological agent that causes disease or illness to its host.
Tumor
A tumor is an abnormal growth or mass of tissue.
Infection
An infection is the detrimental colonization of a host organism by a foreign species.
White blood cell
White blood cells or leukocytes are cells of the immune system which defend the body against both infectious disease and foreign materials.
Antigen
An antigen or immunogen is a molecule that stimulates an immune response.
Antibodies
Antibodies or immunoglobulins are proteins used by the immune system to identify and neutralize foreign objects, such as bacteria and viruses.
Innate immune system
The innate immune system comprises the cells and mechanisms that defend the host from infection by other organisms, in a non-specific manner.
Inflammation
Inflammation is the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants.
Complement system
The 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 system
The adaptive immune response provides the vertebrate immune system with the ability to recognize and remember specific pathogens.
T cell
T 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 immunity
Not 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 cell
Natural killer cells are a form of cytotoxic lymphocyte which constitute a major component of the innate immune system.
Humoral immunity
The humoral immune response is the aspect of immunity that is mediated by antibodies, produced by the cells of the B lymphocyte lineage.
B cell
B cells are lymphocytes that play a large role in the humoral immune response.
Plasma cell
Plasma cells are cells of the immune system that secrete large amounts of antibodies.
Memory B cell
Memory 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
Allergy
Allergy is an abnormal reaction to a substance foreign to the body that is acquired, predictable and rapid.
Allergen
An allergen is a nonparasitic antigen capable of stimulating a type I hypersensitivity reaction in atopic individuals.
Lymph node
Lymph nodes are components of the lymphatic system that contain white blood cells and act as filters or traps for foreign particles.
Lymph vessel
Lymph vessels are thin walled, valved structures that carry lymph.
Neutrophil granulocyte
Neutrophils 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.
Cytokine
Cytokines are a family of small, signaling proteins and glycoproteins particularly important in both innate and adaptive immune responses.
Phagocyte
A phagocyte is a cell that ingests and destroys foreign matter such as microorganisms or debris.
Macrophage
Monocytes and macrophages are phagocytes, acting in both innate immunity as well as cell-mediated immunity of vertebrate animals.
Eosinophil granulocyte
Eosinophil granulocytes are white blood cells of the immune system that are responsible for combating infection by parasites in vertebrates
Antigen presentation
Antigen 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 complex
The 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 receptor
The 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 cell
With 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 cell
A 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 cell
Regulatory 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.
Epitope
An epitope is the part of a macromolecule that is recognized by the immune system, specifically by antibodies, B cells, or T cells.
Thoracic duct
The 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
Spleen
The 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 signalling
Autocrine 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 signalling
Paracrine signaling is a form of cell signaling in which the target cell is close to the signal releasing cell.
Histamine
Histamine is a biogenic amine involved in local immune responses as well as regulating physiological function in the gut and acting as a neurotransmitter.
Mast cell
A mast cell is a resident cell of several types of tissues and contains many granules rich in histamine and heparin.
Monocyte
A 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 I
A primary classes of major histocompatibility complex molecules, MHC class I molecules are found on almost every nucleated cell of the body.
MHC class II
MHC 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 processing
Antigen processing is a biological process that prepares antigens for presentation to special cells of the immune system called T lymphocytes.
Immunoglobulin E
Immunoglobulin 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 G
Built of two heavy chains and two light chains, immunoglobulin G is the most abundant immunoglobulin, constituting 75% of serum immunoglobulins in humans
Monoclonal antibodies
Monoclonal 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 system
The reticuloendothelial system consists of the phagocytic cells located in reticular connective tissue, primarily monocytes and macrophages.
Thymus
The 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.
Interleukin
Interleukins are a group of cytokines that were first seen to be expressed by leukocytes as a means of communication.
Basophil granulocyte
The least common of the granulocytes, basophils degranulate to release histamine, the proteoglycans heparin and chondroitin, and proteolytic enzymes.
Degranulation
Degranulation is a cellular process that releases antimicrobial cytotoxic molecules from secretory vesicles called granules found inside some cells.
Granule
A granule can be any structure barely visible by light microscopy. The term is most often used to describe a secretory vesicle.
Granulocyte
Granulocytes are a category of white blood cells characterised by the presence of granules in their cytoplasm.
Dendritic cell
Dendritic 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 cell
An 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 chain
A heavy chain is the large polypeptide subunit of an antibody.
Light chain
A light chain is the small polypeptide subunit of an antibody.
Polyclonal antibody
Polyclonal antibodies are antibodies that are derived from different B-cell lines.
Autoantibody
An autoantibody is an antibody manufactured by the immune system that is directed against one or more of the individual's own proteins.
Immune tolerance
Immune tolerance is the process by which the immune system does not attack an antigen.
Central tolerance
Central tolerance is the mechanism by which newly developing T cells and B cells are rendered non-reactive to self.
Reticular connective tissue
Reticular 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.
Lymphokine
Lymphokines are a subset of cytokines that are produced by a type of immune cell known as a lymphocyte.
Immunoglobulin superfamily
The immunoglobulin superfamily is a large group of cell surface and soluble proteins that are involved in the recognition, binding, or adhesion processes of cells.
Opsonin
An 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.
Chemokine
Chemokines 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.
Interferon
Interferons (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 burst
Respiratory 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 granules
Specific 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 complex
The membrane attack complex is typically formed on the surface of intruding pathogenic bacterial cells as a result of the activation of the complement system.
Histiocyte
A histiocyte is a monocyte derived tissue macrophage cell.
Microglia
Microglia 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 cells
Gamma/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 region
A [complementarity] determining region is a short amino acid sequence found in the variable domains of antigen receptor proteins that complements an antigen.
VJ recombination
V(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 A
Immunoglobulin A is an antibody which is the main immunoglobulin found in mucous secretions.
Immunoglobulin M
Immunoglobulin 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 allotype
An immunoglobulin allotype is the allele of the antibody chains found in the individual.
Somatic hypermutation
Somatic hypermutation is a mechanism inside cells that diversifies the receptors that the immune system uses to recognize antigens.
Peyer's patches
Peyer'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 storm
A cytokine storm is a potentially fatal immune reaction consisting of a positive feedback loop between cytokines and immune cells.
Leukocyte extravasation
Leukocyte 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 protein
Major 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.
Eicosanoid
Eicosanoids 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.
Prostaglandin
A 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 cell
Kupffer 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 pathway
The 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 cell
A 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.
Granzyme
Granzymes are exogenous serine proteases released by cytoplasmic granules within cytotoxic T cells and natural killer cells.
Perforin
Perforin 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.
Leukotriene
Leukotrienes are naturally produced eicosanoid lipid mediators, which may be responsible for a number of the effects of asthma and allergies.
Lipoxin
Lipoxins are short lived endogenously produced eicosanoids whose appearance in inflammation signals the resolution of inflammation.
Lactoferrin
Found in milk and many mucosal secretions, lactoferrin is a globular multifunctional protein with antimicrobial activity which is part of the innate defense.
Cross-presentation
The 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.
Isotype
The immunoglobulin isotype refers to the type of chain of an antibody.
CD8
CD8 is a transmembrane glycoprotein which serves as a co-receptor for the T cell receptor.
Idiotype
An 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 cell
B-1 cells are B cells that express CD5, which can bind to another B cell surface protein, CD72.
Immunoglobulin class switching
Immunoglobulin class switching is a biological mechanism that changes an antibody from one class to another.
Thymocyte
Thymocytes are T cell precursors which develop in the thymus.
Glycoprotein 130
Glycoprotein 130 is a transmembrane protein which is the founding member of the class of tall cytokine receptors.
Interleukin-1
Interleukin-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 factor
Granulocyte-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-selectin
L-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-gamma
Interferon-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.
Monokine
A monokine is a type of cytokine produced primarily by monocytes and macrophages.
RANTES
Regulated upon Activation, Normal T-cell Expressed, and Secreted, or RANTES, is a protein classified as a chemotactic cytokine or chemokine.
Cathelicidin
Members of the cathelicidin family of antimicrobial polypeptides are characterized by a highly conserved region called the cathelin domain and also a highly variable domain.
Defensin
Consisting 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 pathway
The 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 pathway
Not 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.
CD31
CD31 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.
Superantigen
Produced by certain microbes as a defense mechanism, superantigens are secreted proteins that exhibit highly potent lymphocyte-transforming activity directed towards T lymphocytes.
Paracortex
Paracortex 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 Billroth
The 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.