Mechanism of phagocytosis-mobilization (continued)
Margination-phagocytes adhere to capillary endothelium at inflammatory site.
Diapedesis-phagocytes squeeze through endothelium into tissue spaces at inflammatory site.
2. Engulfment – initially the phagocyte must adhere to the foreign material. In this
adherence process the phagocytes make contact and then stick to the offending
material. This process is aided and abetted by opsonins. Opsonins are proteins
that enhance phagocytosis by coating foreign substances and making them
sticky. Opsonins include antibodies and complement (more later). Pseudopods
then extend around the material to be consumed. The foreign material is taken
into the phagocytic cell in a vesicle known as a phagosome. The phagosome
joins with a lysosome to form a phagolysosome.
3. Digestion and destruction of the pathogen-this occurs within the
phagolysosome. Hydrolytic enzymes within the phagolysosome digest the
microbe. These include proteases, phosphatases, nucleases, lysozyme and
others. Metabolism within the phagocytic cell will switch to anaerobic, which
drops pH and increases the effectiveness of the enzymes. Hydrogen peroxide
will react with chloride anions and produce hypochlorite ions (ClO-). These ions
are very active in their destruction of the microbe.
Some bacteria can still survive this attack. For example, Staphylococcus and Streptococcus produce a
substance called leukocidin. Leukocidin will kill the phagocytic white blood cell.
3. Third line-a specific line of defense also called acquired immunity.
Antigen-any substance that causes antibody formation. It is also referred to as an immunogen. Most
antigens are protein in nature. Antigenic determinant sites are specific regions upon an antigen which
trigger antibody formation. Antigens can have several different antigenic determinant sites. We say
that antigens are therefore polyvalent.
Hapten-a substance that will not cause antibody formation by itself. However, when combined with a
carrier molecule becomes antigenic. The antibiotic penicillin can be considered a hapten. When
penicillin is administered to those who are allergic, penicillin will combine with a serum protein and
become antigenic. This triggers the allergic response.
Antibody-a protein produced in response to an antigen. The antibody will specifically combine with the
antigen that initiated its formation. Antibodies are isolated from the γ (gamma) globulin portion of the
blood serum. The typical antibody consists of two light chains and two heavy chains. Each chain has a
constant and a variable region. The variable regions at the end of the light and heavy chains form a
combining region. Each “Y” shaped antibody has two combining sites. We say that antibodies are
bivalent but monospecific. The two combining sites on a given antibody are the same and will stick to
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