Introduction to Host Defenses and Immunity

This set of vocabulary flashcards covers the fundamental concepts of host defenses, including the three lines of defense, physical and chemical barriers, types of leukocytes, the complement system, inflammation, and fever mechanisms.

Susceptibility

A lack of resistance to a disease, making an individual potentially vulnerable to getting the disease if exposed to a pathogen.

Resistance

The ability to ward off disease; it can be innate (inborn) or adaptive (acquired).

Innate Immunity

Nonspecific resistance that an individual is born with, entailing the first and second lines of defense.

Adaptive Immunity

Specific resistance based on exposure to a specific antigen, also known as acquired immunity.

Antigen

A foreign substance that stimulates an immune response as part of specific resistance.

First Line of Defense

A variety of physical and chemical barriers, such as the skin and mucous membranes, designed to block portals of entry and keep pathogens out of the host.

Second Line of Defense

Internal nonspecific mechanisms including phagocytosis, inflammation, fever, and the complement response.

Third Line of Defense

Specific internal resistance that requires the production of $B$ and $T$ lymphocytes (or cells) initiated by exposure to a foreign antigen.

Keratin

A protective protein that helps protect epithelial layers of the skin and makes them more difficult for pathogens to penetrate.

Sebum

A secretion that keeps skin moist, prevents cracking, and contains fungistatic fatty acids and acidity to inhibit bacterial and fungal growth.

Mucous Membranes

Membranes that secrete mucus, a thick and sticky glycoprotein secretion used as a trap to capture microorganisms.

Ciliary Escalator

A mechanism in the respiratory tract where cilia move trapped microorganisms in mucus up and out of the lungs.

Lacrimal Apparatus

The system that produces tears to wash the surface of the eye, helping to remove microorganisms from a potential portal of entry.

Lysozyme

A digestive enzyme found in perspiration, tears, and saliva that hydrolyzes peptidoglycan in the cell walls of gram-positive cells.

Normal Microbiota

Microorganisms that protect the host by outcompeting pathogens for nutrients and space, or by producing toxins like bacteriocins.

Complement Response

A cascade of more than one blood protein working together through pathways (classical, lectin, or alternative) to prevent bacterial and viral infections.

Classical Pathway

Complement activation initiated when antibodies bind to microorganisms.

Lectin Pathway

Complement activation initiated when blood proteins bind with the carbohydrate mannan found in certain pathogens.

Alternative Pathway

Complement activation where circulating complement proteins bind directly to the outside of a microorganism.

Opsonization

The process of coating a pathogen with complement proteins or antibodies to make it more visible to a phagocyte, enhancing phagocytosis.

Membrane Attack Complex (MAC)

A structure formed by complement proteins in the plasma membrane of a pathogen that creates holes leading to lysis and death.

Granulocytes

A group of leukocytes with visible granules in the cell and lobed nuclei, including neutrophils, eosinophils, and basophils.

Neutrophils

The most numerous circulating leukocytes; they are highly phagocytic, motile, and more active in the initial stages of an infection.

Eosinophils

Granulocytes primarily responsible for destroying eukaryotic parasites and involved in the inflammatory response by producing histamines.

Basophils

Found in low numbers, these granulocytes are extremely important in inflammation and allergic responses because they produce histamines.

Monocyte

An agranulocyte that can mature into a macrophage or a dendritic cell, becoming phagocytic and highly motile.

Antigen Presenting Cell (APC)

A cell such as a macrophage that expresses a consumed foreign antigen on its surface to stimulate $T$ cell activation.

Natural Killer (NK) Cells

Agranulocytes responsible for destroying infected cells or tumor cells that display an altered or missing Major Histocompatibility Complex (MHC).

Apoptosis

Programmed cell death which can be induced by natural killer cells in a targeted host cell.

Diapedesis

The process (also called transmigration) where leukocytes change their shape to weasel through weakened vessel walls into surrounding tissues.

Chemotaxis

Movement in response to a stimulus; in immunity, leukocytes follow a chemical breadcrumb trail of cytokines to an infection site.

Phagolysosome

The structure formed by the fusion of a phagosome (containing a pathogen) and a lysosome (containing digestive enzymes).

Leukocidins

Compounds produced by pathogens like Staphylococcus aureus that kill phagocytes.

Erythema

Redness of the skin associated with inflammation caused by increased blood flow due to vasodilation.

Pyrogen

A compound that resets the hypothalamus to elevate body temperature, causing a fever; can be exogenous (like Lipid A) or endogenous (like IL-1).

Hydrophores

Pathogen structures that steal iron from the host; their activity is inhibited in high fever situations.

Antipyretic Drugs

Medications like aspirin or acetaminophen that lower fever by blocking the activity of pyrogens on the hypothalamus.

Spleen

An organ that filters the blood to remove dead red blood cells, leukocytes, cellular debris, and toxins.

Cytokines

Chemical messengers produced by immune cells to communicate, coordinate, and stimulate different aspects of the immune response.

Interferons

Cytokines produced by viral-infected cells or leukocytes that induce surrounding cells to produce antiviral proteins.