The immune process works like this: an infectious agent enters the body. Maybe it’s a virus that infects us or maybe it’s a bacterium. The immune system is always alert to detect and attack the infectious agent. Whatever the agent, the immune system recognizes it as a foreign body.
These outer bodies are called antigens. The body’s first line of defense is a group of cells called macrophages. These cells circulate through the bloodstream and in the tissues of the body, vigilant of the antigens.
When an invader enters, a macrophage quickly detects it and captures it inside the cell. Enzymes inside the macrophage destroy the antigen by processing it into small pieces called antigenic peptides.
Sometimes this process alone is enough to eliminate the invader. However, in most cases, other cells of the immune system must join the fight. But before other cells can begin their work, the antigenic peptides within the macrophage bind to molecules called human leukocyte antigens or HLA.
The peptide-bound HLA molecule, now called the antigen complex, is released from the macrophage.
Cells called T-class lymphocytes can then recognize and interact with the HLA antigenic peptide complex found on the surface of the macrophage.
Once this complex is recognized, T lymphocytes send chemical signals called cytokines. These cytokines attract more T lymphocytes. They also alert other lymphocytes, class B, to produce antibodies.
These antibodies are released into the bloodstream to find and bind more antigens, so that invaders cannot multiply and make you sick. In the last step of this process, a cell called phagocyte is responsible for removing the antigen from the body.