acellular
lacking cells
Viruses are acellular, parasitic entities that are not classified within any domain because they are not considered alive.
apoptosis
the cell death caused by induction of a cell’s own internal mechanisms either as a natural step in the development of a multicellular organism or by other environmental factors such as signals from cells of the immune system
After the NK cell detects an infected or tumor cell, it induces programmed cell death, or apoptosis.
attenuation
the weakening of a virus during vaccine development
Live vaccines are usually made by attenuation (weakening) of the “wild-type” (disease-causing) virus by growing it in the laboratory in tissues or at temperatures different from what the virus is accustomed to in the host.
capsid
the protein coating of the viral core
All virions have a nucleic-acid genome covered by a protective layer of protein, called a capsid.
cytopathic
causing cell damage
These changes, called cytopathic effects, can change cell functions or even destroy the cell.
glycoprotein
a protein molecule with attached carbohydrate molecules
All viruses use some sort of glycoprotein to attach to their host cells at molecules on the cell called viral receptors.
vaccine
a weakened solution of virus components, viruses, or other agents that produce an immune response
For example, the fact that influenza is an RNA virus is one reason a new flu vaccine is needed every year.
virion
an individual virus particle outside a host cell
Virions, single virus particles, are very small, about 20–250 nanometers (1 nanometer = 1/1,000,000 mm).
viral envelope
a lipid bilayer that envelops some viruses
The outer structure surrounding the capsid of some viruses is called the viral envelope.
complement system
an array of approximately 20 soluble proteins of the innate immune system that enhance phagocytosis, bore holes in pathogens, and recruit lymphocytes
These include the inflammatory response, phagocytosis, natural killer cells, and the complement system.
cytokine
a chemical messenger that regulates cell differentiation, proliferation, and gene expression to effect immune responses
When a pathogen is recognized as foreign, chemicals called cytokines are released.
inflammation
the localized redness, swelling, heat, and pain that results from the movement of leukocytes through opened capillaries to a site of infection
The second line of defense of the innate system includes chemical signals that produce inflammation and fever responses as well as mobilizing protective cells and other chemical defenses.
innate immunity
an immunity that occurs naturally because of genetic factors or physiology, and is not caused by infection or vaccination
Innate immunity is not caused by an infection or vaccination and depends initially on physical and chemical barriers that work on all pathogens, sometimes called the first line of defense.
interferon
a cytokine that inhibits viral replication
One class of early-acting cytokines is the interferons, which are released by infected cells as a warning to nearby uninfected cells.
lymphocyte
a type of white blood cell that includes natural killer cells of the innate immune system and B and T cells of the adaptive immune system
A lymphocyte is a white blood cell that contains a large nucleus (Figure 12.10).
macrophage
a large phagocytic cell that engulfs foreign particles and pathogens
For example, a monocyte is a type of white blood cell that circulates in the blood and lymph and develops into a macrophage after it moves into infected tissue.
major histocompatibility class (MHC) I
a group of proteins found on the surface of all nucleated cells that signals to immune cells whether the cell is normal or is infected or cancerous; it also provides the appropriate sites into which antigens can be loaded for recognition by lymphocytes
NK cells identify intracellular infections, especially from viruses, by the altered expression of major histocompatibility class (MHC) I molecules on the surface of infected cells.
mast cell
a leukocyte that produces inflammatory molecules, such as histamine, in response to large pathogens
Mast cells are produced in the same way as white blood cells, but unlike circulating white blood cells, mast cells take up residence in connective tissues and especially mucosal tissues.
monocyte
a type of white blood cell that circulates in the blood and lymph and differentiates into a macrophage after it moves into infected tissue
For example, a monocyte is a type of white blood cell that circulates in the blood and lymph and develops into a macrophage after it moves into infected tissue.
natural killer (NK) cell
a lymphocyte that can kill cells infected with viruses or tumor cells
These include the inflammatory response, phagocytosis, natural killer cells, and the complement system.
neutrophil
a phagocytic leukocyte that engulfs and digests pathogens
For example, a neutrophil is an early arriving white blood cell that engulfs and digests pathogens.
white blood cell
a nucleated cell found in the blood that is a part of the immune system; also called leukocytes
White blood cells in the blood and lymph recognize pathogens as foreign to the body.
active immunity
an immunity that occurs as a result of the activity of the body’s own cells rather than from antibodies acquired from an external source
The production of antibodies by plasma cells in response to an antigen is called active immunity and describes the host’s active response of the immune system to an infection or to a vaccination.
adaptive immunity
a specific immune response that occurs after exposure to an antigen either from a pathogen or a vaccination
The adaptive, or acquired, immune response takes days or even weeks to become established—much longer than the innate response; however, adaptive immunity is more specific to an invading pathogen.
antibody
a protein that is produced by plasma cells after stimulation by an antigen; also known as an immunoglobulin
The plasma cells, on the other hand, produce and secrete large quantities, up to 100 million molecules per hour, of antibody molecules.
antigen
a macromolecule that reacts with cells of the immune system and which may or may not have a stimulatory effect
Adaptive immunity is an immunity that occurs after exposure to an antigen either from a pathogen or a vaccination.
antigen-presenting cell (APC)
an immune cell that detects, engulfs, and informs the adaptive immune response about an infection by presenting the processed antigen on its cell surface
Then, an antigen-presenting cell (APC) detects, engulfs, and informs the adaptive immune response about an infection.
B cell
a lymphocyte that matures in the bone marrow
Activated T and B cells whose surface binding sites are specific to the molecules on the pathogen greatly increase in numbers and attack the invading pathogen.
cell-mediated immune response
an adaptive immune response that is controlled by T cells
B cells are involved in the humoral immune response, which targets pathogens loose in blood and lymph, and T cells are involved in the cell-mediated immune response, which targets infected cells.
cytotoxic T lymphocyte (TC)
an adaptive immune cell that directly kills infected cells via enzymes, and that releases cytokines to enhance the immune response
Cytotoxic T cells (TC) are the key component of the cell-mediated part of the adaptive immune system and attack and destroy infected cells.
dendritic cell
an immune cell that processes antigen material and presents it on the surface of its cell in MHC class II molecules and induces an immune response in other cells
Instead, dendritic cells and macrophages first engulf and digest pathogens into hundreds or thousands of antigens.
effector cell
a lymphocyte that has differentiated, such as a B cell, plasma cell, or cytotoxic T cell
B plasma cells and TC cells are collectively called effector cells because they are involved in “effecting” (bringing about) the immune response of killing pathogens and infected host cells.
helper T lymphocyte (TH)
a cell of the adaptive immune system that binds APCs via MHC class II molecules and stimulates B cells or secretes cytokines to initiate the immune response
However, if the host is re-exposed to the same pathogen type, circulating memory cells will immediately differentiate into plasma cells and TC cells without input from APCs or TH cells.
humoral immune response
the adaptive immune response that is controlled by activated B cells and antibodies
B cells are involved in the humoral immune response, which targets pathogens loose in blood and lymph, and T cells are involved in the cell-mediated immune response, which targets infected cells.
immune tolerance
an acquired ability to prevent an unnecessary or harmful immune response to a detected foreign body known not to cause disease
The acquired ability to prevent an unnecessary or harmful immune response to a detected foreign substance known not to cause disease, or self-antigens, is described as immune tolerance.
lymph
the watery fluid present in the lymphatic circulatory system that bathes tissues and organs with protective white blood cells and does not contain erythrocytes
White blood cells in the blood and lymph recognize pathogens as foreign to the body.
memory cell
an antigen-specific B or T lymphocyte that does not differentiate into an effector cell during the primary immune response but that can immediately become an effector cell on reexposure to the same pathogen
As B and T cells mature into effector cells, a subset of the naïve populations differentiates into B and T memory cells with the same antigen specificities (Figure 12.17).
major histocompatibility class (MHC) II molecule
a protein found on the surface of antigen-presenting cells that signals to immune cells whether the cell is normal or is infected or cancerous; it provides the appropriate template into which antigens can be loaded for recognition by lymphocytes
NK cells identify intracellular infections, especially from viruses, by the altered expression of major histocompatibility class (MHC) I molecules on the surface of infected cells.
passive immunity
an immunity that does not result from the activity of the body’s own immune cells but by transfer of antibodies from one individual to another
Some of the mother’s immunoglobulins will pass through the placenta, providing passive immunity against some potential infections.
primary immune response
the response of the adaptive immune system to the first exposure to an antigen
Vaccination is based on the knowledge that exposure to noninfectious antigens, derived from known pathogens, generates a mild primary immune response.
secondary immune response
the response of the adaptive immune system to a second or later exposure to an antigen mediated by memory cells
This is known as the secondary immune response.
T cell
a lymphocyte that matures in the thymus gland
The two types of lymphocytes of the adaptive immune response are B and T cells (Figure 12.12).
allergy
an immune reaction that results from immediate hypersensitivities in which an antibody-mediated immune response occurs within minutes of exposure to a harmless antigen
The immune reaction that results from immediate hypersensitivities in which an antibody-mediated immune response occurs within minutes of exposure to a usually harmless antigen is called an allergy.
autoantibody
an antibody that incorrectly marks “self” components as foreign and stimulates the immune response
An antibody that inappropriately marks self-components as foreign is termed an autoantibody.
autoimmunity
a type of hypersensitivity to self-antigens
Autoimmunity is a type of hypersensitivity to self-antigens that affects approximately five percent of the population.
hypersensitivity
a spectrum of inappropriate immune responses toward harmless foreign particles or self-antigens; occurs after tissue sensitization and includes immediate-type (allergy), delayed-type, and autoimmunity
A maladaptive immune response toward harmless foreign substances or self-antigens that occur after tissue sensitization is termed a hypersensitivity.
immunodeficiency
a failure, insufficiency, or delay at any level of the immune system, which may be acquired or inherited
Many animal viruses, such as HIV (human immunodeficiency virus), leave the infected cells of the immune system by a process known as budding, where virions leave the cell individually.
These terms were extracted from Concepts of Biology – 1st Canadian Edition by Charles Molnar and Jane Gair, eCampusOntario (originally published by BCcampus) which is licensed under a Creative Commons Attribution 4.0 International License. Chapter content adapted from Concepts of Biology by OpenStax (© Rice University), licensed under CC BY 4.0.