An antigen cell is a cell with fragments of antigens and histocompatibility complexes on its surface. These cells circulate through the body and present the fragments to T-cells, which learn to recognize specific antigens and can trigger responses when they appear in the body. Such cells are an important part of the immune system, and they become more complex over time in response to antigen exposure. This can be deliberate through vaccination as well as accidental, as a result of interacting with sick people or handling contaminated objects.
Also known as antigen presenting cells, these cells are capable of engulfing antigens and processing them into the fragments that they display on their surface. Some antigen cell types include B lymphocytes and phagocytic cells such as macrophages. They float through the body, interacting with T-cells that can lock onto the antigen fragments and histocompatibility complex. In an active infection, they tend to cluster in the lymph nodes.
On their own, T-cells cannot recognize most antigens. They rely on antigen cells to process the antigens and repackage them in a format that the T-cells will be able to identify. Likewise, an antigen cell cannot trigger an immune reaction on its own. If something is wrong with a patient's T-cells, the antigen cells might function normally but have no T-cells to lock onto, so they cannot prompt the immune system to fight an infection.
An antigen cell can be a tumor marker in some cases. A protein known as carcinoembyronic antigen, for example, is associated with many kinds of cancers. In a developing fetus, levels of this protein spike, but in adults, they should be very low. If they are unusually high, it suggests that the patient might have a cancerous growth, such as colon cancer. This protein can help a cancer spread, and cells with the antigen will bind more easily to tissues in the body.
A doctor might be able to check for the presence of some antigen cell types with the use of a blood test. This can determine whether a patient has been exposed to an antigen and whether his or her body formed an immune response. The doctor can take a small sample of blood and send it to a lab for screening to check for antigen cells and determine their concentration in the blood.