Acquired (adaptive or specific) immunity is not present at birth. It is learned. As a person's immune system encounters foreign substances (antigens), the components of acquired immunity learn the best way to attack each antigen and begin to develop a memory for that antigen. Acquired immunity is also called specific immunity because it tailors its attack to a specific antigen previously encountered. Its hallmarks are its ability to learn, adapt, and remember. Acquired immunity takes time to develop after initial exposure to a new antigen. However, because a memory is formed, subsequent responses to a previously encountered antigen are more effective.
Lymphocytes are the type of white blood cell responsible for acquired immunity. Typically, an acquired immune response begins when antibodies, produced by B cells (B lymphocytes), encounter an antigen. Dendritic cells, cytokines, and the complement system (which enhances the effectiveness of antibodies) are also involved.
Lymphocytes enable the body to remember antigens and to distinguish self from harmful nonself (including viruses and bacteria). Lymphocytes circulate in the bloodstream and lymphatic system and move into tissues as needed.
The immune system can remember every antigen encountered because after an encounter, some lymphocytes develop into memory cells. These cells live a long time—for years or even decades. When these cells encounter an antigen for the second time, they recognize it immediately and respond quickly, vigorously, and specifically to that particular antigen. This specific immune response is the reason that people do not contract chickenpox or measles more than once and that vaccination can prevent certain disorders.
Lymphocytes may be T cells or B cells.
T cells are produced in the thymus. They can potentially recognize an almost limitless number of different antigens. To avoid attacking the body's own tissues, they need to learn how to distinguish self from nonself antigens. Normally, only the T cells that ignore the body's own antigens (self antigens) are allowed to mature and leave the thymus. Without this training process, T cells could attack the body's cells and tissues.
Mature T cells are stored in secondary lymphoid organs (lymph nodes, spleen, tonsils, appendix, and Peyer patches in the small intestine). These cells circulate in the bloodstream and the lymphatic system. After they first encounter an infected or abnormal cell, they are activated and search for those particular cells.
There are different types of T cells:
When T cells initially encounter an antigen, most of them perform their designated function, but some of them develop into memory cells, which remember the antigen and respond to it more vigorously when they encounter it again.
Sometimes T cells—for reasons that are not completely understood—do not distinguish self from nonself. This malfunction can result in an autoimmune disorder, in which the body attacks its own tissues (see see Autoimmune Disorders).
B cells are formed in the bone marrow. B cells have particular sites (receptors) on their surface where antigens can attach. B cells can learn to recognize an almost limitless number of different antigens.
The B-cell response to antigens has two stages:
Although the main function of B cells is to produce antibodies, they can also present antigen to T cells.
When a B cell encounters an antigen, it is stimulated to mature into a plasma cell or a memory B cell. Plasma cells then release antibodies (also called immunoglobulins, or Ig). Antibodies protect the body in the following ways:
Antibodies are essential for fighting off certain types of bacterial and fungal infections. They can also help fight viruses.
Antibodies attach to the antigen and form an immune complex (antibody-antigen complex). The antibody and antigen fit tightly together, like pieces of a jigsaw puzzle. Sometimes an antibody can attach to other antigens if the antigens closely resemble the antigen that the antibody was formed to recognize and attach to.
|Basic Y Structure of Antibodies
An antibody molecule is basically shaped like a Y. The molecule has two parts:
Variable part: This part varies from antibody to antibody, depending on which antigen the antibody targets. The antigen attaches to the variable part.
Constant part: This part can be one of five structures, which determines the antibody's class— IgM, IgG, IgA, IgE, or IgD. This part is the same within each class.
Each antibody molecule has two parts. One part varies. It is specialized to attach to a specific antigen. The other part is one of five structures, which determines the antibody's class—IgM, IgG, IgA, IgE, or IgD. This part is the same within each class and determines the function of the antibody.
This class of antibody is produced when a particular antigen (such as an antigen of an infectious microorganism) is encountered for the first time. The response triggered by the first encounter with an antigen is the primary immune response. IgM then attaches to the antigen, activating the complement system, and thus makes the microorganism easier to ingest.
Normally, IgM is present in the bloodstream but not in the tissues.
IgG, the most prevalent class of antibody, is produced in greater amounts when a particular antigen is encountered again. More antibody is produced in this response (called the secondary immune response) than in the primary immune response. The secondary immune response is also faster and the antibodies produced—mainly IgG—are more effective. IgG protects against bacteria, viruses, fungi, and toxic substances.
IgG is present in the bloodstream and tissues. It is the only class of antibody that crosses the placenta from mother to fetus. The mother's IgG protects the fetus and infant until the infant's immune system can produce its own antibodies. Also, IgG is the most common class of antibody used in treatment.
These antibodies help defend against the invasion of microorganisms through body surfaces lined with a mucous membrane, including those of the nose, eyes, lungs, and digestive tract. IgA is present in the bloodstream, in secretions produced by mucous membranes (such as tears and saliva), and in colostrum (the fluid produced by the breasts during the first few days after delivery, before breast milk is produced).
These antibodies trigger immediate allergic reactions (see see Overview of Allergic Reactions). IgE binds to basophils (a type of white blood cell) in the bloodstream and mast cells in tissues. When basophils or mast cells with IgE bound to them encounter allergens (antigens that cause allergic reactions), they release substances (such as histamine) that cause inflammation and damage surrounding tissues. Thus, IgE is the only class of antibody that often seems to do more harm than good. However, IgE helps defend against certain parasitic infections that are common in some developing countries.
Small amounts of IgE are present in the bloodstream and mucus of the digestive system. These amounts are higher in people with asthma, hay fever, other allergic disorders, and parasitic infections.
IgD is present mainly on the surface of immature B cells. It helps these cells mature. Small amounts of these antibodies are present in the bloodstream. Their function in the bloodstream, if any, is not well understood.
Last full review/revision March 2013 by Peter J. Delves, PhD