The immune system has many components:

Antibodies (immunoglobulins) are proteins that are produced by white blood cells called B cells and that tightly bind to the antigen of an invader, tagging the invader for attack or directly neutralizing it. The body produces thousands of different antibodies. Each antibody is specific to a given antigen.

Antigens are any substance that the immune system can recognize and that can thus stimulate an immune response.

B cells (B lymphocytes) are white blood cells that produce antibodies specific to the antigen that stimulated their production.

Basophils are white blood cells that release histamine (a substance involved in allergic reactions) and that produce substances to attract other white blood cells (neutrophils and eosinophils) to a trouble spot.

Cells are the smallest unit of a living organism, composed of a nucleus and cytoplasm surrounded by a membrane.

Chemotaxis is the process of by which a chemical substance attracts cells to a particular site.

The complement system consists of a group of proteins that are involved in a series of reactions (called the complement cascade) designed to defend the body—for example, by killing bacteria and other foreign cells, making foreign cells easier for macrophages to identify and ingest, and attracting macrophages and neutrophils to a trouble spot.

Cytokines are numerous different proteins that are secreted by immune and other cells and that act as the immune system’s messengers to help regulate an immune response.

Dendritic cells are derived from white blood cells. They reside in tissues and help T cells recognize foreign antigens.

Eosinophils are white blood cells that kill bacteria and other foreign cells too big to ingest, and they may help immobilize and kill parasites and help destroy cancer cells. Eosinophils also participate in allergic reactions.

Helper T cells are white blood cells that help B cells produce antibodies against foreign antigens, help killer T cells become active, and stimulate macrophages, enabling them to ingest infected or abnormal cells more efficiently.

Histocompatibility (literally, compatibility of tissue) is determined by human leukocyte antigens (self-identification molecules). Histocompatibility is used to determine whether a transplanted tissue or organ will be accepted by the recipient.

Human leukocyte antigens (HLA) are a group of identification molecules located on the surface of all cells in a combination that is almost unique for each person, thereby enabling the body to distinguish self from nonself. This group of identification molecules is also called the major histocompatibility complex.

An immune complex is an antibody attached to an antigen.

An immune response is the reaction of the immune system to an antigen.

Immunoglobulin is another name for antibody.

Interleukin is a type of messenger (cytokine) secreted by some white blood cells to affect other white blood cells.

Killer (cytotoxic) T cells are T cells that attach to infected cells and cancer cells and kill them.

Leukocyte is another name for a white blood cell, such as a monocyte, a neutrophil, an eosinophil, a basophil, or a lymphocyte (a B cell or T cell).

The lymphatic system is a network of lymph nodes connected by lymphatic vessels that helps the body transport microorganisms and dead or damaged cells to be filtered out and destroyed. Acquired immune responses are initiated in the lymph nodes.

Lymphocytes are the white blood cells responsible for acquired (specific) immunity, including producing antibodies (by B cells), distinguishing self from nonself (by T cells), and killing infected cells and cancer cells (by killer T cells).

Macrophages are large cells that develop from white blood cells called monocytes. They ingest bacteria and other foreign cells and help T cells identify microorganisms and other foreign substances. Macrophages are normally present in the lungs, skin, liver, and other tissues.

Major histocompatibility complex (MHC) is a synonym for human leukocyte antigens.

Mast cells are cells in tissues that release histamine and other substances involved in inflammatory and allergic reactions.

A molecule is a group of atoms chemically combined to form a unique substance.

Natural killer cells are a type of white blood cell that can recognize and kill abnormal cells, such as certain infected cells and cancer cells, without having to first learn that the cells are abnormal.

Neutrophils are white blood cells that ingest and kill bacteria and other foreign cells.

Phagocytes are a type of cell that ingests and kills or destroys invading microorganisms, other cells, and cell fragments. Phagocytes include neutrophils and macrophages.

Phagocytosis is the process of a cell engulfing and ingesting an invading microorganism, another cell, or a cell fragment.

A receptor is a molecule on a cell’s surface or inside the cell that can identify specific molecules, which fit precisely in it—as a key fits in its lock.

Regulatory (suppressor) T cells are white blood cells that help end an immune response.

T cells (T lymphocytes) are white blood cells that are involved in acquired immunity. There are three types: helper, killer (cytotoxic), and regulatory.