MCAT Biology - The Immune System

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immune system

contain and eliminate infections

Innate/nonspecific immunity

defenses that are always active against infection, but lack the ability to target specific invaders; acts near entry points into the body

Adaptive/specific immunity

defenses that target a specific pathogen; slower to act, but can maintain immunological memory of an infection to mount a faster attack in subsequent infections

bone marrow

produces leukocytes through the process of hematopoiesis

leukocytes (white blood cells)

participate in the immune system

spleen

location of blood storage and activation of B-cells

B-cells

turn into plasma cells to produce antibodies as part of adaptive immunity; binding of antigen causes activation

plasma cells

produce antibodies

humoral immunity

antibodies dissolve and act in the blood rather than within cells; driven by B-cells and antibodies

T-cells

coordinate the immune system and directly kill virally infected cells

cell-mediated immunity

T-cells

thymus

a small gland just in front of the pericardium, the sac that protects the heart; matures T-cells

lymph nodes

major component of the lymphatic system; provide a place for immune cells to communicate and mount an attack; can activate B-cells

gut-associated lymphoid tissue (GALT)

Other immune tissue found in close proximity to the digestive system, which is a site of potential invasion by pathogens

tonsils

gut-associated lymphoid tissue in the throat

adenoids

gut-associated lymphoid tissue in the sinus

Peyer’s patches

gut-associated lymphoid tissue in the small intestine

appendix

contains lymphoid aggregates; attached to cecum

hematopoiesis

development and differentiation of blood cells

hematopoietic stem cells

make leukocytes and other blood cells

granulocytes

leukocytes containing granules

agranulocytes

leukocytes not containing granules

granules

contain toxic enzymes and chemicals, which can be released by exocytosis, and are particularly effective against bacterial, fungal, and parasitic pathogens

neutrophils

type of phagocytic white blood cell; part of innate immunity; form the most abundant type of granulocytes; very short lived (~5 days); target bacteria via chemotaxis/opsonisation; stained by neutral dyes

eosinophils

type of white blood cell; part of innate immunity; bright red-orange granules; combat parasites and infections; release large amounts of histamine; associated with allergies; stained by acidic dyes

basophils

type of white blood cell; part of innate immunity; large purple granules; responsible for inflammatory responses; the least populous leukocyte; associated with allergies; stained by basic dyes

lymphocytes

responsible for antibody production, immune system modulation, and targeted killing of infected cells

Monocytes

phagocytic cells in the bloodstream

macrophages

monocytes in tissues; phagocytizes the invader through endocytosis, digests the invader using enzymes, presents little pieces of the invader (mostly peptides) to other cells using MHC; release cytokines

microglia

macrophages in nervous system

Langerhans cells

macrophages in skin

osteoclasts

macrophages in bone

skin (integument)

provides a physical barrier between the outside world and our internal organs, preventing most bacteria, viruses, fungi, and parasites from entering the body

defensins

antibacterial enzymes found on the skin

Sweat

antimicrobial properties

mucous membranes

lined with cilia to trap particulate matter; prevent bacteria and viruses from gaining access to sensitive tissues; respiratory system - push it up toward the oropharynx, where it can be swallowed or expelled

lysozyme

nonspecific bacterial enzyme secreted in tears and saliva

stomach

secretes acid, resulting in the elimination of most pathogens

gut/GI tract

colonized by bacteria that lack the necessary characteristics to cause infection; potential invaders are not able to compete with large population and are thus kept at bay

complement sytem

consists of a number of proteins in the blood that act as a nonspecific defense against bacteria; punch holes in the cell walls of bacteria, making them osmotically unstable

classical pathway

requires the binding of an antibody to a pathogen to activate complements

alternative pathway

does not require antibodies to activate complements

interferons

proteins that prevent viral replication and dispersion produced by cells that have been infected with viruses; cause nearby cells to decrease production of both viral and cellular proteins; decrease the permeability of these cells; upregulate MHC class I and class II molecules, resulting in increased antigen presentation and better detection of the infected cells by the immune system; responsible for many “flu-like” symptoms that occur during viral infection, including malaise, tiredness, muscle soreness, and fever

resident population

permanent, rather than transient, cell group of macrophages in the tissue

major histocompatibility complex (MHC)

binds to antigen and carries it to the cell surface, where it can be recognized by cells of the adaptive immune system;

antigen

a substance (usually a pathogenic protein) that can be targeted by an antibody

cytokines

chemical substances that stimulate inflammation and recruit additional immune cells to the area

MHC class I

All nucleated cells in the body; Any protein produced within a cell can be loaded and presented on the surface; allows the immune system to monitor the health of these cells and to detect if the cells have been infected with a virus or another intracellular pathogen; infected cells can then be killed by cytotoxic T-cells to prevent infection

endogenous pathway

MHC-I pathway; binds antigens that come from inside the cell

MHC class II

mainly displayed by professional antigen-presenting cells; presentation may result in the activation of both the innate and adaptive immune systems

professional antigen-presenting cells

phagocytic cells pick up pathogens from the environment, process them, and then present them

macrophages, dendritic cells in the skin, some B-cells, and certain activated epithelial cells

exogenous pathway

MHC-II pathway; antigens originated outside the cell

pattern recognition receptors (PRR),

special receptors on macrophages and dendritic cells that recognize the category of the invader (bacterium, virus, fungus, or parasite); allows for the production of appropriate cytokines to recruit the right type of immune cells

toll-like receptors (TLR)

class of pattern recognition receptors (PRRs) that recognize structurally conserved molecules derived from microbes, commonly known as pathogen-associated molecular patterns (PAMPs)

Natural killer (NK) cells

nonspecific lymphocyte able to detect the downregulation of MHC and induce apoptosis in these virally infected cells; also target cancer cells that can also downregulate MHC production

opsonized

marked with an antibody from a B-cell

pus

dead neutrophil collections as result of infection

histamine

inflammatory mediator leading to inflammation

inflammation

vasodilation and increased leakiness of the blood vessels, allowing additional immune cells (especially macrophages and neutrophils) to move out of the bloodstream and into the tissue; particularly useful against extracellular pathogens

mast cells

closely related to basophils, but have smaller granules and exist in the tissues, mucosa, and epithelium; release large amounts of histamine in response to allergens, leading to inflammatory responses

antibodies / immunoglobulins [Ig]

produced by B-cells; present on the surface of a cell or secreted into body fluids; binds to antigens; Y-shaped molecules; five different isotypes (IgM, IgD, IgG, IgE, and IgA)

agglutinate

cause pathogens to clump together; forming large insoluble complexes that can be phagocytized

degranulation

exocytosis of granule contents; antigen binds to antibodies on the surface of a mast cell; releasing histamine and causing an inflammatory allergic reaction

heavy chains

inner part of antibody linked to C domain

light chains

outer part of antibody bonded by disulfide linkages and noncovalent interactions to the larger part

antigen-binding region

specific polypeptide sequences that will bind one, and only one, specific antigenic sequence at the tips of the Y

variable region (domain)

part of the antibody that changes in each type of antibody

variable region (domain)

each B-cell changes its antigen-binding region, trying to find the best match for the antigen

clonal selection

mechanism for generating antibody specificity

constant region (domain)

region that cells such as natural killer cells, macrophages, monocytes, and eosinophils have receptors for, and that can initiate the complement cascade

isotype switching

Cells can change which isotype of antibody they produce when stimulated by specific cytokines

naïve B-cells

have not yet been exposed to an antigen

Plasma cells

daughter B-cell; produce large amounts of antibodies; eventually die

memory B-cells

daughter B-cell; stay in the lymph node, awaiting reexposure to the same antigen; may last the lifetime of the organism

primary response

initial activation of B-cells takes approximately seven to ten days

secondary response

if same microbe is ever encountered again, the memory cells rapidly proliferate and differentiate into plasma cells to produce antibodies specific to that pathogen

vaccination

biological preparation that provides active acquired immunity to a particular infectious or malignant disease; prophylactic (pre-infection) or therapeutic (post-infection)

Positive selection

allowing only the maturation of cells that can respond to the presentation of antigen on MHC

Negative selection

causing apoptosis in cells that are self-reactive

thymosin

a peptide hormone secreted by thymic cells that fosters maturation of T-cells

Helper (T_h)/CD4⁺ T-cells

coordinate the immune response by secreting chemicals known as lymphokines; respond to antigens presented on MHC-II molecules; most effective against bacterial, fungal, and parasitic infections

lymphokines

capable of recruiting other immune cells (such as plasma cells, cytotoxic T-cells, and macrophages) and increasing their activity

human immunodeficiency virus (HIV)

causes loss of helper T-cells; prevents the immune system from mounting an adequate response to infection

acquired immunodeficiency syndrome (AIDS)

advanced HIV infection; even weak pathogens can cause devastating consequences as opportunistic infections

Cytotoxic T-cells (T_c) / CD8⁺ T-cells (CTL)

capable of directly killing virally infected cells by injecting toxic chemicals that promote apoptosis into the infected cell; respond to antigens presented on MHC-I molecules; most effective against viral (and intracellular bacterial or fungal) infections

Suppressor / regulatory T-cells (T_reg)

express CD4 and Foxp3; help to tone down the immune response once infection has been adequately contained

self-tolerance

turn off self-reactive lymphocytes to prevent autoimmune diseases

memory T-cells

lie in wait until the next exposure to the same antigen until they carry out a more robust and rapid response

Self-antigens

proteins and carbohydrates present on the surface of every cell of the body; signal to immune cells that the cell is not foreign and should not be attacked

autoimmunity

immune system attacks cells expressing particular self-antigens

hypersensitivity reactions

immune systems are hypersensitive to antigens that are not inherently threating and become overactivated when these antigens are encountered; allergies and autoimmunity

glucocorticoids

modified versions of cortisol; potent immunosuppressive qualities

active immunity

immune system is stimulated to produce antibodies against a specific pathogen

ex. infection, prophylactic vaccine

Passive immunity

transfer of antibodies to an individual

ex. fetal immunity, infant immunity

lymphatic system

type of circulatory system; equalizes of Fluid Distribution; transports biomolecules; immunity

lymph / lymphatic fluid

fluid leaked from blood vessels that is picked up and circulated by lymphatic system

thoracic duct

connection of lymphatic and cardiovascular system; delivers the fluid into the left subclavian vein near the heart in the posterior chest

Lymph nodes

small, bean-shaped structures along the lymphatic vessels; contain a lymphatic channel, as well as an artery and a vein; provide a space for the cells of the immune system to be exposed to possible pathogens

edema

swelling due to fluid collecting in tissue; when lymphatics are overwhelmed

Lacteals

small lymphatic vessels; located at the center of each villus in the small intestine; fats, packaged into chylomicrons by intestinal mucosal cells travel through