Specific Host Defenses

Flashcards covering the vocabulary and concepts of Specific Host Defenses, including antigens, MHC, antibody classes, lymphocyte development, and types of immunity.

Immunocompetence

The ability of the body to react with foreign substances.

Antigen

Molecules (primarily proteins or sugars) that stimulate a response by B and T cells; they must not be a normal constituent of the body.

Epitope

The specific portion of an antigen to which an immune cell responds; it serves as the primary signal to the immune system.

Cell-mediated immunity

The adaptive immune response involving the activation of T-lymphocytes.

Humoral immunity

The adaptive immune response involving the activation of B-lymphocytes and the production of antibodies.

Major Histocompatibility Complex (MHC)

A set of markers found on all nucleated cells (excluding red blood cells) that is critical for the recognition of self and the rejection of foreign tissue.

Class I MHC

Molecules that appear on all nucleated cells and display epitopes of self.

Class II MHC

Molecules found on macrophages, dendritic cells, and B cells (antigen-presenting cells) that present antigens to T cells.

Class III MHC

Molecules involved with the complement system.

Antigen-presenting cells (APCs)

Cells such as macrophages, dendritic cells, and B cells that engulf microbes, degrade them, and transport antigen pieces to their membrane complexed with MHC-II receptors.

Superantigens

Bacterial toxins that serve as potent stimuli for T cells, causing $100\times$ greater activation and an overwhelming release of cytokines.

CD (Cluster of Differentiation)

Surface markers on lymphocytes used for identification; examples include CD3, CD4, and CD8.

Immunoglobulin (Ig)

Large glycoproteins that serve as the antigen receptors of B cells and, when secreted, are known as antibodies.

Opsonization

A process where antibodies coat the surface of a bacterium to stimulate more efficient recognition and engulfment by phagocytes.

Neutralization

A reaction where antibodies fill the surface receptors on a virus or the active site of a microbial enzyme to prevent normal attachment.

Agglutination

The capacity for antibodies to cross-link cells or particles into large clumps, rendering microbes immobile and enhancing phagocytosis.

Antitoxin

A special type of antibody that neutralizes bacterial exotoxins.

IgA

An antibody class that exists as a monomer in circulation and a dimer in secretions, providing immunity against enteric, respiratory, and genitourinary pathogens; also found in colostrum.

IgM

A large pentamer or hexamer antibody produced during the primary response; it is "sticky" and serves as a powerful complement activator.

IgG

The most common antibody in circulation ($75$\%); it is the only antibody capable of crossing the placenta and is associated with secondary immune responses.

IgE

An antibody class associated with responses to parasites and allergic reactions.

Plasma cells

Progeny of activated B cells that function as "antibody factories."

Clonal selection

The process by which a specific B or T cell is activated by a single incoming antigen for which it is pre-programmed.

Clonal expansion

The rapid division and production of B or T cells after they encounter their specific antigen.

Helper T cells (TH)

T cells (CD4+) that activate macrophages, assist B-cell processes, and help activate cytotoxic T cells.

Regulatory T cells (Treg)

T cells that serve to limit inflammation and prevent autoimmune responses.

Cytotoxic T cells (TC)

T cells (CD8+) that destroy virally infected cells, cancer cells, and "foreign" cells from other humans or animals by inducing apoptosis or using the perforin pathway.

Natural Killer (NK) cells

Lymphocytes that lack antigen specificity and attack cells lacking MHC markers; they are often the first to attack cancer and virus-infected cells.

Perforins

Molecules released by TC or NK cells that polymerize and form holes in the membrane of a target cell, leading to lysis.

Natural immunity

Immunity acquired through normal biological experiences, such as recovering from an infection or receiving maternal antibodies.

Artificial immunity

Immunity acquired purposefully through medical procedures, such as vaccines or immune serum.

Active immunity

Resistance produced when an antigen activates B and T cells, creating memory; it requires several days to develop but is long-lasting.

Passive immunity

Instantaneous, short-term protection acquired by receiving antibodies from another human or animal; it lacks memory and antibody manufacturing capability.

Adjuvant

A special binding substance used in some vaccines to enhance immunogenicity, prolong retention at the injection site, and facilitate contact with APCs.

Herd immunity

A status where a high percentage of vaccinated individuals protects the individual and common good by decreasing the likelihood of outbreaks.