Immune System Flashcards

Flashcards about the Immune System and the Body's Defense

Immune System

Protects us from infectious agents and harmful substances, typically without our awareness. It is composed of numerous cellular and molecular structures that function together to provide us with immunity, dependent on the specific type of infectious agent.

Infectious Agents

Organisms that cause damage or death to a host organism. If they cause harm, they are termed pathogenic. Five major categories include bacteria, viruses, fungi, protozoans, and multicellular parasites.

Cytokines

Small soluble proteins produced by cells of both the innate and adaptive immune system. They are released from one cell and bind specific receptors of a target cell and have a short half-life.

Cytokine Functions

Regulate and facilitate immune system activity, serve as means of communication between cells, control behavior of effector cells of immunity, regulate inflammatory response, serve as weapons to destroy cells, and influence non-immune cells.

Innate vs Adaptive Immune System

Two categories of the immune system: innate immunity and adaptive immunity. They are organized based on the type of immunity provided and work together to protect from harmful agents.

Innate Immunity

Provided by multiple components that protect against a wide array of substances. It is non-specific and includes skin, mucosal membranes, most immune cells such as macrophages and NK cells, chemicals such as interferon and complement, and physiologic responses like inflammation and fever.

Adaptive Immunity

Provided by lymphocytes that are activated to replicate and respond when stimulated by a specific antigen. Includes T-lymphocytes (cell-mediated immunity), B-lymphocytes (antibody-mediated immunity), and plasma cells (synthesize and release antibodies).

Innate Immunity Structures

Structures prevent entry of potentially harmful substances and respond nonspecifically to a wide range of harmful substances. It is the first line of defense (external) and second line of defense (internal).

Skin

Physical barrier formed by the epidermis and dermis that few microbes are able to penetrate. It releases antimicrobial substances and has normal flora that help prevent growth of pathogenic microorganisms.

Mucosal Membrane Barrier

Membranes lining the openings of the body that produce mucin, which when hydrated forms mucus. Lined by harmless bacteria (GI tract) which suppress the growth of more virulent types.

Second Line of Defense

Nonspecific internal defenses that include selected immune cells, antimicrobial proteins, inflammation, and fever.

Neutrophils and Macrophages

Engulf unwanted substances through phagocytosis. The phagosome, a vesicle containing the unwanted substance, merges with a lysosome to form a phagolysosome. Substances are digested by digestive enzymes from the lysosome. Performs respiratory burst to release reactive oxygen containing molecules.

Basophils & Mast Cells

Proinflammatory chemical-secreting cells. Basophils circulate in the blood, and mast cells reside in connective tissue, mucosa, and internal organs.

Natural killer cells

Destroy a wide variety of unwanted cells, including virus- and bacteria-infected cells, tumor cells, and cells of transplanted tissue. They are formed in bone marrow and circulate in blood and accumulate in secondary lymphatic structures. Patrol the body detecting unhealthy cells termed immune surveillance.

Eosinophil

Attack multicellular parasites by degranulating and releasing enzymes and other toxic substances, and can release proteins that form transmembrane pores in the parasite’s cells.

Complement System

Important group of antimicrobial proteins composed of at least 30 plasma proteins identified with the letter “C”. Synthesized by the liver, continuously released in inactive form, and activated in the blood by enzyme cascade. Especially potent against bacterial infections.

Inflammation

Immediate, local, nonspecific response that occurs in vascularized tissue against a variety of stimuli and is a major effector response of innate immunity. It helps eliminate most infectious agents from body.

Fever

Abnormal elevation of body temperature (at least 1°C from normal (37°C) and may accompany inflammatory response. Events include pyrogens circulating through blood and targeting hypothalamus, hypothalamus raising temperature set point leading to fever stages onset, stadium, and defervescence.

Pus and Abscesses

Exudate that contains destroyed pathogens, dead leukocytes, macrophages, and cellular debris, removed by lymphatic system or through skin. If not completely cleared, may form abscess – pus walled off with collagen fibers. Usually requires surgical intervention to remove.

Chronic Inflammation

Inflammation continuing for longer than two weeks, characterized by macrophages and lymphocytes (not neutrophils). Can occur from overuse injuries or if acute inflammation is unable to eliminate pathogen, or may be due to autoimmune disorder. Can lead to tissue destruction and scar tissue formation.

Adaptive Immunity

Initiated upon entry of foreign substance and takes longer to respond than innate immunity. Contact with antigen causes lymphocyte to proliferate and form specialized “army.” Lymphocytes and products released are collectively termed immune response and it's considered the third line of body’s defense.

Two branches of Adaptive Immunity

Cell-mediated immunity involving T-lymphocytes and antibody-mediated immunity involving B-lymphocytes, plasma cells, and antibodies.

Antigens

Substances that bind to a component of adaptive immunity. Usually a protein or large polysaccharide, including protein capsid of viruses, cell wall of bacteria or fungi, bacterial toxins, and tumor cells with unique abnormal protein.

Foreign antigens

Different in structure from human body’s molecules that bind body’s immune components

Self-antigens

Body’s molecules that typically do not bind immune components. In some cases , the immune system reacts to self-antigens as if foreign, resulting in autoimmune disorders.

Antigenic Determinant

Also known as epitope, the specific site on antigen recognized by immune system. Each has a different shape. Pathogenic organisms can have multiple determinants

Haptens

Molecules too small to function as antigen alone, are antigenic when attached to carrier molecule in host. Account for hypersensitivity reactions to drugs, pollen, snake venom.

Antigen Presentation

Display of an antigen on a cell’s plasma membrane. Process performed by other cells to help T-lymphocytes “see” the antigen

Antigen-presenting cells (APCs)

Immune cells that present to both helper T-cells and cytotoxic T-cells (dendritic cells, macrophages, B- lymphocytes). Attachment of antigen to major histocompatibility complex (MHC) is required.

MHC class I molecules

Glycoproteins that have a genetically determined structure that is unique to individual. Continuously synthesized and modified by RER then inserted into cell membrane. Display fragments of proteins that were bound in RER

MHC class II molecules

Also glycoproteins that are synthesized and modified by RER and sent to membrane. Exogenous antigens are brought into cell through endocytosis, the substance is digested into peptide fragments, and the fragments are loaded onto MHC class II molecules within the vesicle then displayed on cell membrane.

Overview of Life Events of Lymphocytes

Three main events include the formation of lymphocytes in primary lymphoid structures, the activation of lymphocytes in secondary lymphoid structures, and the effector response at the site of infection.

Formation of T-lymphocytes

T-lymphocytes originate in red bone marrow and migrate to thymus as pre-T-lymphocytes to complete maturation. Each cell has its TCR “tested” through a process of selection for whether it can bind MHC with antigen and whether it binds only foreign (“nonself ”) antigen.

Positive selection

Selects for the ability of T-cells to bind thymic epithelial cells with MHC molecules (those that can bind survive)

Negative selection

Tests ability of T-lymphocyte to AVOID binding self-antigens (self-tolerance).

Immunocompetent + Naive T-lymphocyte

Able to bind antigen and respond to it, but not yet exposed to antigens they recognize.

Regulatory T-lymphocytes (Tregs)

CD4+ cells formed from T-cells that bind self-antigens, inhibit immune response, and prevent lymphocytes from recognizing self-antigens as foreign .

Clonal selection + Antigen challenge

Forming clones in response to an antigen. Antigen challenge is the first encounter between antigen and lymphocyte (usually in secondary lymphoid structures).

Activation of Helper T-lymphocytes

Require activation before carrying out immune system functions. First signal is direct physical contact between APC and helper T-lymphocyte. Second signal is other receptors of APC and T-cell interact. Helper T-cell secretes interleukin-2, stimulating itself. Lack of a second signal results in helper T-cells becoming Tregs.

Activation of Cytotoxic T-lymphocytes

First signal: Direct contact between TCR of cytotoxic T-cell and peptide fragment with MCH I molecule (on APC or infected cell). Second signal: Other receptors of APC and T-cell interact. IL-2 released from helper T-cells binds to and stimulates cytotoxic T-lymphocytes

Activation of B-Lymphocytes

First signal is intact antigen binds to BCR, cross-linking 2 BCRs. Stimulated B-cell engulfs, processes, and presents antigen to helper T-cell for recognition. Second signal is receptors between B-cell and a previously activated T-cell interact; Helper T-cell releases IL-4, stimulating B-lymphocyte

Effector Response

Mechanisms activated lymphocytes use to help eliminate antigen include the regulation of cells of adaptive and innate immunity via release of cytokines (helper T-lymphocytes), destroying unhealthy cells by apoptosis (cytotoxic T-lymphocytes), and producing antibodies (plasma cells)

Effector response of helper T-lymphocytes

Leave secondary lymphatic structure after exposure to antigen, migrate to site of infection, continue to release cytokines to regulate other immune cells, help activate B-lymphocytes, activate cytotoxic T-lymphocytes, and enhance formation and activity of innate immune system cells.

Effector response of cytotoxic T- lymphocytes

Leave secondary lymphatic structure after exposure to antigen, migrate to site of infection, and destroy infected cells that display the antigen. Physical contact is made between cytotoxic cell and unhealthy or foreign cell

Effector Response of B-Lymphocyte

Form antibodies, the effectors of humoral immunity. Antibodies circulate through body in lymph and blood and ultimately come in contact with antigen. Antibody titer is the circulating blood concentration of antibody against specific antigen

Antibodies

Immunoglobulin proteins produced against a particular antigen facilitating destruction by other immune cells. They are Y-shaped, soluble proteins referred to as humoral immunity.

IgG

75-85% of antibodies in blood and predominant antibody in lymph, cerebrospinal, serous, and peritoneal fluids. They can participate in all previously mentioned antibody actions and can cross the placenta and cause hemolytic disease of the newborn Know These

IgM

Normally a pentamer found mostly in blood, it is most effective at agglutination and binding complement. Responsible for rejection of mismatched transfusions

IgA

Found in areas exposed to environment, produced in mucus, saliva, tears, breastmilk. Protects respiratory and GI tract. It is dimer composed of two antibody molecules; helps prevent pathogens adhering to epithelial tissue and is especially good at agglutination.

IgD

Functions as antigen-specific B-lymphocyte receptor and identifies when immature B-lymphocytes are ready for activation.

IgE

Blood that has a low rate of synthesis, formed in response to allergic reactions and parasitic infections, causes release of products from basophils and mast cells, and attracts eosinophils.

Adaptive Immunity Activation

Requires direct contact between lymphocyte and antigen. Antigen is recognized by limited numbers of T- and B-lymphocytes. There is a Lag time between exposure and direct contact with lymphocytes and memory cells are formed in response to lymphocyte activation.

Immunologic Memory

With subsequent antigen exposure, many memory cells make contact with antigen more rapidly and produce more powerful response termed secondary response. Vaccines are typically effective in developing memory and secondary response is seen on exposure to substance vaccinated against.

Active Immunity

Results from direct encounter with pathogen through direct exposure to antigen or vaccine, to help produce memory cells against specific antigen .

Passive Immunity

Obtained from another individual via transfer of antibodies from mother to fetus or serum containing antibodies transferred from one person to another. Neither form produces memory cells and lasts only as long as antibody proteins are present.

Vaccinations

Weakened or dead microorganism or component that stimulates immune system to develop memory B- lymphocytes. Provide relatively safe means for initial exposure to microorganism. If later exposed, secondary response triggered and immune system response predominantly from the humoral branch.

Hypersensitivities

Overreaction of immune system noninfectious substance (allergen). Can cause multiple symptoms, such as runny nose and watery eyes, red welts and itchy skin (hives), labored breathing and coughing (allergic asthma).

HIV and AIDS

Life-threatening condition that results from human immunodeficiency virus, which infects and destroys helper T-lymphocytes. Numbers dropping to low numbers, the virus resides in body fluids of infected individuals and be transmitted by intercourse, needle sharing, breastfeeding. Prevention includes safe sex