Actin is a globular protein found in all higher organisms. It makes up a significant part of the cytoskeleton — or backbone of the cell — and is involved in cell movement. Antibodies are responses of the immune system to an antigen, which is generally a foreign object. Each individual organism can have millions of antibodies that react against different antigens. In some cases, such as autoimmune diseases, the organism makes antibodies against its own cellular components.
An actin antibody is one that reacts against one of the forms of actin, which is highly similar between different species of organisms. Its active form is as filaments. This is how it comprises part of the cytoskeleton and is involved in helping cells and muscles contract. There are three main groups in vertebrate cells — alpha, beta, and gamma. Alpha actins are generally found in muscle. Beta and gamma actins are found in most types of cells.
An actin antibody typically falls into two categories of interest. The first is comprised of those antibodies made in animals to be used in laboratory research. The other category is autoimmune diseases in which an anti-actin antibody is produced by humans.
An example of the latter category is a smooth muscle antibody. The presence of this type of human antibody is characteristic for people with Type I autoimmune liver disease, a chronic liver disorder. Its presence is used as part of the diagnosis for the disease. Patients with hepatitis also have smooth muscle antibodies. These types of antibodies are directed against a type of alpha actin, known as smooth muscle actin.
Another autoimmune disorder associated with an actin antibody is Celiac disease. The presence of these antibodies in the blood has been found to reliably correlate with intestinal damage in these patients. Celiac disease is caused by a reaction to a gluten protein in wheat, which disrupts the absorption of nutrients in the small intestine.
Experimentally, an actin antibody is often used as a control for experiments utilizing antibodies. Since actin is so structurally similar between species, one can be fairly sure that an actin antibody from one species will react with another. Loading controls of beta actin ensure that the same amount of sample has been loaded in each lane of a protein gel. This is used during electrophoresis, in which one separates proteins by running an electrical current through a gel. The proteins are then transferred to a membrane, and probed with antibodies in what is called a Western Blot.
Fluorescently-labeled actin antibodies are used in many cell biology experiments that study the movement of actin in the cytoskeleton and in internal cell structures, such as vesicles. Such actin antibodies can be monitored with specialized microscopy known as immunofluorescence microscopy. Since so many activities of the cell require actin, these types of experiments are performed across a wide array of disciplines in cell biology.