Who's Who?
Antibody: A protein (immunoglobulin) produced by a B-cell that binds to a specific foreign antigen in the blood or body fluids. This leads to attack by the immune system.

Antibodies are grouped into one of five immunoglobulin classes:
IgA: The predominant antibody present on most mucosal surfaces; it exists in mucosal secretions as secretory IgA (sIgA).

IgG: The most common serum antibody. It can pass from mother to fetus during pregnancy, and diffuse across mucosal surfaces. IgG is also present in vaginal secretions, in higher concentrations than sIgA.

IgM: An antibody made locally in mucosal-associated lymphatic tissue. It is produced early in the immune response to a pathogen, before IgG.

IgD: An antibody present on the B-lymphocyte membrane. IgD's function is unknown.

IgE: A trace serum protein (antibody) associated with allergic reactions.

Activation: A process by which an immune system cell is "switched on" to deal with an infection.

Anergy: Lack of response by the immune system. A T-cell that meets a foreign molecule (antigen) and doesn't respond is described as anergic.

Antigen: Any piece of a substance (foreign protein, toxin, bacteria) that can induce a specific immune response or react to a specific antibody or T-cell.

Antigen-Presenting Cell (APC): A cell that can process an infectious agent and present pieces of it (antigens) to T-cells. Dendritic cells and macrophages function as APCs.

Costimulation: A signaling mechanism that helps control the T-cell response to infections. In order to respond to an antigen, a T-cell needs to receive several signals. The first signal is the docking of the antigen with the T-cell receptor. Antigen-presenting cells also deliver secondary signals to the T-cell to enhance its response. These secondary signals involve interactions of molecules on the surface of the antigen-presenting cell and the T-cell. It is these secondary signals that scientists refer to as costimulation of the T-cell.

CD4 Helper T-Cell: A type of T-lymphocyte that acts as the conductor of the immune system orchestra. Provides vital help to other immune system cells.

CD8 T-Cell: A type of T-lymphocyte that can have several different functions in the immune system. CD8 cytotoxic T-lymphocytes (CD8 CTLs) can kill cells in the body that have been infected by a foreign organism. Other CD8 cells can release chemicals that suppress the replication of infectious agents.

Cellular immunity: Primary immune response involving T-lymphocytes (T-cells), particularly killer T-cells (or cytotoxic T-lymphocytes, CTLs).

Dendritic cell: A specialized cell that functions as an antigen-presenting cell for T-cells.

Epitope: Specific amino acid sequence of a foreign antigen, such as a virus, that an antibody recognizes, binds to, and reacts against.

Homeostasis: A Greek word used in biology to describe the balancing act required to keep a complicated system functioning. For the immune system, homeostasis refers to maintaining the balance of different cell types (naive, memory, B-cells, CD4 T-cells, CD8 T-cells) required for healthy immune function.

Humoral immunity: Secondary immune response involving B-lymphocytes (B-cells) that produce antibodies.

Immune system: The combination of cellular, molecular, and genetic components that provide a defense against foreign substances (antigens), including cells, bacteria, viruses, proteins, and parasites.

Lymphatic system: A network of vessels in the body for transporting immune system cells that is separate from the blood circulation. Includes the lymph nodes, which are immune system command centers.

Lymphocytes: Cells of the immune system that respond specifically to foreign antigens. There are two classes of lymphocytes: B-lymphocytes (B-cells) and T-lymphocytes (T-cells). They are found in blood, lymph, and lymphoid tissues. B-cells become memory or plasma cells. T-cells differentiate into several types: helper (CD4 T-cells); suppressor (CD4 and CD8 T-cells); cytotoxic (CD4 and CD8 T-cells); and memory (CD4 and CD8-T-cells).

Monoclonal antibody: A pure antibody that recognizes only one epitope. It is isolated from a single B-cell and grown in cell culture.

Mucosal immunity: Immune defenses provided by the mucous membranes of the body, a one-cell layer thick moist barrier that contains mucosal cells, B-cells, and T-cells.

Neutralizing antibody: Antibody that stops a virus from replicating by lysis (killing) of infected cells.

Passive immunization: Administering antibodies for temporary immune protection against an antigen. Immunity exists as long as the antibodies remain in the body, usually days or weeks.

Pathogenic: Causes disease.

Polyclonal antibodies: A mixed population of antibodies that recognize numerous epitopes.

Resting cell: An immune cell capable of responding to an infection that is not currently active.

Systemic immunity: Affecting the body as a whole; all-body immunity.

T-Cell Receptor (TCR): A structure on the surface of every T-cell that acts as a docking bay for antigens. A T-cell has around 20,000 copies of the same receptor dotted all over its surface.

Vaccination: Administering an antigen to generate long-term antibody or cellular immune responses or both.

- Richard Jefferys, Vicki Burkitt