MSD Manual

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Specialized Cells and Molecules of the Immune System

Specialized Cells and Molecules of the Immune System


Small white blood cells found in all organs and tissues. There are 2 principal types: B lymphocytes (B cells), which mature in the bone marrow, and T lymphocytes (T cells), which mature in the thymus.

B cells

These lymphocytes are responsible for the production of antibodies, an important part of the immune response.

T cells

These lymphocytes include killer (cytotoxic) T cells, which detect and kill cells that are abnormal (such as cancer cells), and helper T cells, which help other lymphocytes mount an immune response.


Any substances that can induce an immune response.

Antigen-presenting cells

Specialized cells that engulf antigens and process them so that they can be recognized by lymphocytes.


All-purpose scavenger cells that ingest and destroy antigens and cell debris.


White blood cells that stimulate healing and remove antigens, injured cells, and neutrophils from tissue.


White blood cells that ingest bacteria and other foreign cells, participate in allergic reactions, and help destroy cancer cells.

Mast cells

Cells that release histamine and other substances involved in allergic reactions.


The immune system’s messengers, typically small proteins, which help regulate an immune response by delivering signals from one cell to another.


Also called immunoglobulins, antibodies are proteins produced by B cells that interact with specific antigens. They can form immune complexes, label antigens for removal by other cells, or block the ability of a virus to enter its target cell.