Antibodies and Macrophages Attack the Antigens Help
Introduction to Antigens
Most cells carry chemical markers upon their surface membranes that uniquely identify them. Thus, cells in a particular human body have their own chemical surface markers that mark them as “self.” Such cells are not attacked by the body’s immune system. Foreign cells transplanted from some other human body, or from a bacterium or cancer cell, however, have different surface markers that mark them as “non-self.” The general name for such surface markers is antigens ( AN -tih- jens ).
The word, antigen, means “produced” (-gen) “against” (anti-). This meaning reflects the fact that antigens are “non-self” marker proteins that label particular cells as foreign. Therefore, the antigens are foreign proteins that cause antibodies ( AN -tih- bah -dees) to be “produced against” them. Antibodies, in turn, are proteins produced by the body’s immune system that attack and destroy foreign antigens. The overall process is called an antigen-antibody reaction.
The antigen-antibody reaction comes about after a series of preceding steps (Figure 17.2). The first step is identification of a foreign cell and its surface antigen by a thymic ( THIGH -mik) lymphocyte ( LIMPF -oh- sight ) within the body tissues. The thymic lymphocyte is also abbreviated as T-lymphocyte (or as a T-cell, mentioned back in Chapter 6). The T-lymphocytes prowl around within the extensive network of the reticuloendothelial system and act much like scouts. They send out a chemical signal whenever a foreign antigen is encountered.
The bone marrow or B-lymphocytes receive the chemical messages from the T-lymphocyte scouts. The B-lymphocytes then undergo a marked differentiation (process of becoming specialized or different). They transform into plasma cells. Plasma cells have a prominent “clock face” nucleus when viewed through a compound light microscope. There is dark chromatin (kroh- MAT -in) visible – strands of DNA that have not yet coiled together to create chromosomes. These chromatin fragments are arranged in a circular fashion around the edges of the nucleus, giving it a distinct “clock face” appearance.
It is the plasma cells that actually produce the antibodies. Once produced, the individual antibody molecules attach to the foreign antigens, like two pieces of a jigsaw puzzle fitted together. The result we have called an antigen-antibody reaction. When the antibody combines, it causes a lysis (breakdown) of the invading cell carrying the foreign antigen. Thus, millions of invading or abnormal cells (and their antigens) are efficiently ruptured and scattered into tiny pieces.
Moving nearby is a defensive army of wandering macrophages ( MAH -kroh- fah -jes), or “large” (macr) “eating” (phag) cells. Such wandering macrophages often include the monocytes ( MAHN -oh- sights ). The monocytes are a type of leukocyte that has a “single” (mono-), large, horseshoe-shaped nucleus. The monocyte can creep out of the blood in capillaries by making amoeboid (amoeba-like) movements through their extremely thin walls. They enter the surrounding tissue and extend their cytoplasm like twin arms or pseudopodia. The monocytes readily surround and engulf entire invading cells and their foreign antigens by means of phagocytosis (cell eating).
Hence, by two major processes, antigen-antibody reactions and phagocytosis, a state of immunity from disease is usually accomplished within the human body.
Practice problems for these concepts can be found at: Immune and Lymphatic Systems Test
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