Sometimes, the body relies on a specific defense against a particular antigen (usually a protein molecule that the body recognizes as foreign). Antigens occur on pathogens, but they can also be part of a foreign cell or a cancerous cell. Ordinarily, a person does not become immune to his or her body's normal cells; therefore, the immune system is able to distinguish "self' from "nonself".
Immunity usually lasts a long time. For example, once a person has had measles, he or she usually cannot be infected by the measles virus a second time. Immunity is primarily the result of the action of B lymphocytes and T lymphocytes. B, which stands for bone marrow, lymphocytes mature in the bone marrow, and T, which stands for thymus, lymphocytes mature in the thymus gland. B lymphocytes, also called B cells, become plasma cells that produce antibodies, proteins capable of combining with and inactivating antigens. These antibodies are secreted into the blood, lymph, and mucus. In contrast, T lymphocytes, also called T cells, do not produce antibodies. Instead, certain T cells directly attack cells bearing antigens they recognize. Other T cells regulate the immune response.
Lymphocytes can recognize antigens because they have receptor molecules on their surfaces. The shape of the receptors on any particular lymphocyte is appropriate to a portion of one specific antigen. The receptor and antigen are said to fit together like a lock and key. During a person's lifetime, the body is estimated to encounter 1 million different antigens; therefore, it needs 1 million different lymphocytes with specific receptors for protection against those antigens. Despite this great diversity, none of the lymphocytes ordinarily attacks the body's own cells.
Two types of lymphocytes provide specific immunity. B cells produce and secrete antibodies that combine vvith antigens. Certain T cells directly attack antigen-bearing cells. and others regulate the immune response.
Antibody-Mediated Immunity
The receptor on a B cell is called a membrane-bound antibody because antibodies are the secreted form of the B-cell receptor. A B cell is activated when it encounters a bacterial cell or a toxin bearing an appropriate antigen; that is, the B cell has the potential to divide many times and produce many plasma cells that will secrete antibodies against this antigen. All of the plasma cells derived from one parent B lymphocyte are called clones, and a clone produces one type of antibody. A B cell does not clone until its antigen is present. The clonal selection theory states that the antigen selects the B cell that will produce a clone of plasma cells.
Once antibody production is sufficient, the antigen disappears from the system, and the development of plasma cells ceases. However, some members of a clone do not participate in antibody production; instead, they remain in the bloodstream as memory B cells, which are capable of producing the antibody specific to a particular antigen for some time, even as long as a lifetime in some instances. As long as these cells are present, the individual is said to be actively immune- that is, future antibody production is possible because the memory B cells can form into more plasma cells if the same antigen invades the system again.
Defense by B cells is called antibody-mediated immunity, or humoral immunity, because B cells produce antibodies that are present in the bloodstream.
B cells are responsible for antibody-mediated immunity. Each one produces a specific type of antibody to counteract a particular infection.
Each antibody contains variable regions that bind to an antigen in a lock-and-key manner and a constant region. Antibodies can be classified according to their constant regions. Most antibodies in the blood belong to the class IgG (immunoglobulin G).
The antigen-antibody reaction can take several forms, but frequently, the reaction produces complexes of antigens combined with antibodies. When viruses and bacterial toxins have combined with specific antibodies, they cannot attach to target cells. An antigen-antibody complex, sometimes called the immune complex, marks the antigen for destruction by other forces. For example, the complex may be engulfed by neutrophils or macrophages, or it may activate complement. In this case, complement makes pathogens more susceptible to phagocytosis.
An antibody combines vvith its antigen in a lock-and-key manner. The antibody-antigen reaction can lead to complexes that contain several molecules of antibody and antigen. These complexes are then destroyed by phagocytic cells or complement.
Cell-Mediated Immunity
T cells are responsible for cell-mediated immunity. Their receptors "see" an antigen held in a cleft formed by a major histocompatibility (MHC) protein 1 that lies on the T-ceL surface. The importance of MHC proteins was first recognized when researchers discovered that they contribute to specificity of tissues and make it difficult to transplant tissue from one person to another. In other words, the donor and recipiend must be histo (tissue) compatible for a transplant to be successful.
T cells are activated by macrophages that "present" a complex consisting of the antigen within an MHC protein to them. Langerhans cells also present antigens to T cells. Langerhans cells occur in the skin and are responsible for initiating a defense against bacteria that have penetrated the outermost layers of epidermis and against superficial skin cancers.
After being activated, some T cells, called helper T cells, stimulate B cells. They also release lymphokines, "messenger" proteins that stimulate the immune system. The AIDS virus attacks helper T cells, which accounts for the fact that a person with AIDS has an impaired immune system.
Other types of T cells divide and become "killer cells."
Killer cells exhibit cytotoxicity, meaning that direct contact between a killer cell and a target cell (bearing a specific antigen) causes death of the target cell. Killer T cells specialize in providing resistance against foreign antigens. They attack any tissue cell that bears a foreign antigen, including virus-infected cells, the cells of a transplanted organ or donated skin graft, or cancer cells. Cancer cells are abnormal cells that most likely display altered antigens. As long as T cells are capable of recognizing newly developed cancer cells, cancer cannot grow or spread. When a T cell attacks a cell, it releases chemicals that perforate the cell's membrane, causing the cell to burst before shriveling up and dying.
T cells are responsible for cell-mediated immunity. They directly attack cells that bear antigens. For example, they protect the body against cancer but also attack transplanted organs.
