• This glossary provides a list of definitions for common terms that are important in understanding innate immunity
  • The words in blue are defined elsewhere in the document

Adaptive immunity/adaptive immune response: one of two arms of the immune system,and the second to be stimulated afterinfection. Activation of the adaptive immune response occurs only after stimulation of the innate immune response. An adaptive immune response takes a number of days to develop and is characterized by antibody production and activation of T cells, which recognize highly specific targets. Responses are strengthened with each subsequent exposure to an antigen. This is known as immunological memory.1

Antibody: a protein produced by plasma cells of the adaptive immune system in response to an infection. Antibodies have a binding site with a unique structure that is specific to a particular antigen. Their function is to bind pathogens and neutralize them or flag them for destruction by phagocytes.2

Antigen: any molecule that can bind to an antibody or a T-cell receptor, the latter leading to production of antibodies. The range of molecules with antigenic properties is diverse and includes not only molecules associated with pathogens, but also those derived from the animal itself. For this reason, antigens initially only elicit immune responses when encountered in the context of an infection i.e. when there is stimulation of innate immunity. However, following this initial response to a particular antigen, subsequent exposure to the same antigen can elicit an immune response, even in the absence of innate stimulation.2

B cell: a type of adaptive immune cell (lymphocyte) that differentiates into antibody-secreting plasma cells when activated.1

Chemokines: protein molecules produced during an immune response that act as chemoattractants (substances that induce cells to move towards them), guiding cells such as phagocytes to the site of an infection.2

CpG DNA: a particular sequence of nucleotide bases that is found more commonly in bacterial than in mammalian or avian DNA. In bacteria, this type of DNA is unmethylated, whereas the host CpG DNA is methylated. This causes receptors to recognize it as non-self, and confers its immunostimulatory properties.3,4

CpG oligodeoxynucleotide (ODN): short synthetic sequences ofDNA bases (oligodeoxynucleotides) that contain the CpG DNA sequence. This sequence is analogous to the unmethylated CpG DNA found in bacterial or viral DNA. CpG ODNs are capable of stimulating an immune response, because they can bind TLR9/TLR21, causing activation of innate immune cells. 4-6

Cytokines: a group of protein molecules secreted mainly by immune cells in response to infectious stimuli. Cytokines are signaling molecules that can be detected by other immune cells. There are many different types of cytokines that induce a range of different responses in the target cell and their effects can be either immune stimulatory or inhibitory. They are central to the coordination of an immune response.1

Dendritic cells: cells that link innate and adaptive immunity. They have phagocytic properties and can take up pathogens in the tissues. Upon recognition of pathogenic structures through their toll-like receptors, the dendritic cell matures and develops features that enable it to activate cells of the adaptive immune system.2

DNA methylation: the addition of methyl groups to DNA, usually at cytosine nucleoside bases. Methyl groups are organic compounds that play an important role in cellular processes by binding to DNA and regulating gene expression.7 DNA methylation is common in vertebrates but unusual in bacterial or viral DNA.3,8

Immune response: An immune response is stimulated when a pathogen gets past the animal's physical barriers which are the body’s first line of defense against attack. The immune response occurs in two phases, known as innate and adaptive immunity that work together to eliminate the threat.2

Immune system: a collection of molecules, cells and processes that act together in a coordinated effort to recognize and eliminate infectious microorganisms.1

Inflammatory response: a response to infection aimed at fighting off the pathogen and removing foreign materials from the body. Upon recognition of an infectious microorganism, immune cells are recruited to the site of infection and activated by signaling molecules. Inflammation can result in clinical signs, including redness, heat, swelling and pain, which are caused by the infiltration of cells to the area and production of inflammatory mediators.1,2

Innate immunity/innate immune response: the first of the two arms of the immune system to react to the presence of infection. Innate immune cells can distinguish the animal’s own cells from pathogens, and when a foreign invader is identified, they rapidly instigate an innate immune response. This is characterized by phagocytosis, whereby the infecting pathogen is destroyed and removed, and mobilization of other immune cells to the site of infection to amplify the response. A crucial feature of an innate immune response is its ability to convey a danger signal to cells of the adaptive immune system, which will respond later with a stronger and more specific attack on the specific pathogen present.1

Interferon: a type of cytokine, classified into two different groups. Type 1 interferons are produced in response to viral infection and their actions help inhibit viral replication. The primary functions of type 2 interferons include macrophage activation, and regulation of the adaptive immune response.2

Liposome: lipid structures that take a spherical shape and can be used as carriers to introduce drugs or immune stimulants, such as CpG DNA, into cells. The liposome helps to direct its contents to the appropriate compartment within a cell, where it can exert its effects. Liposomes are made up of positively charged lipids, sometimes incorporating neutral lipids; the net positive charge represents a danger signal that is recognized by immune cells. Liposomes can thus stimulate an immune response in their own right, as well as modifying the immune response elicited by their contents.9,10

Macrophage: a type of innate immune cell that expresses pathogen recognition receptors (e.g. toll-like receptors), allowing it to detect infection and subsequently relay this information to other immune cells through the production of signaling molecules known as cytokines. Macrophages are phagocytes that can engulf particles, allowing them to come into contact with intracellular toll-like receptors.2

Pathogen: microorganisms that can enter and replicate within the tissues of their animal host and cause infection. Two important types of pathogen in animal disease are bacteria and viruses. Pathogens usually have specialized mechanisms for overcoming host barriers to infection.1

Phagocyte: a type of immune cell that specializes in destroying pathogens. There are several types of phagocyte, including macrophages, neutrophils and dendritic cells. Macrophages and neutrophils belong to the innate immune system and are concerned with destroying and clearing the pathogen they have ingested. Dendritic cells are unique in that they bridge the innate and adaptive immune responses.2

Phagocytosis: a process carried out by cells known as phagocytes, whereby they internalize pathogens and break them down with enzymes.2

T cell: a type of adaptive immune cell (lymphocyte) that mediates cytotoxic responses against cells infected with viruses, and stimulates other lymphocytes (B cells) to produce antibodies and activate phagocytes to destroy other microorganisms.1

Toll-like receptors (TLRs): a type of receptor found on innate immune cells. There are a number of different TLRs that recognize a variety of pathogenic components (PAMPs). When the receptor binds to the particular molecule it recognizes, known as its ligand, a signaling cascade is initiated that leads to cellular activation.2,6

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