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Phagocytosis and Immunity: Mechanisms and Processes, Study notes of Microbiology

The process of phagocytosis, a crucial part of the body's immune system, and the role of immunity in protecting against pathogens. Topics include the steps of phagocytosis, the function of opsonins, and the formation of antibodies as part of the humoral immune response.

Typology: Study notes

2011/2012

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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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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.

  1. 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.
  2. 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.

  1. 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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the same antigen. Antibodies act extracellularly, they are secreted by B lymphocytes (B cells) that have differentiated into plasma cells. A given B cell has the capability of ultimately producing antibodies with combining sites for a particular antigen. To adequately deal with all of the antigenic challenges in life there must be many different B cells.

Active immunity-an immune response which involves the process of one’s own body actively manufacturing antibodies.

Passive immunity-an immune response which uses pre-made antibodies produced outside of the body.

Natural immunity-immunity gained from natural processes.

Artificial immunity-immunity gained from scientific intervention.

Active and natural immunity-one’s own body manufacturing antibodies after the natural exposure to an infectious agent. Symptoms may be clinical or subclinical (not showing overt symptoms).

Active and artificial immunity-one’s own body manufacturing antibodies in response to a vaccine. Vaccines contain killed, living and attenuated microorganisms/infectious agents which act as antigens to produce immunity. Toxoids, which are toxins inactivated by heat can also produce an active and artificial immunity. Parts of infectious agents can also be antigenic and be used in vaccines.

Passive and natural immunity-passing of maternal antibodies through the placenta to the fetus is an example of this type of immunity. IgG (immunoglobulin G) is the class of antibody passed along in this fashion. Also, antibodies can be acquired by the newborn through the colostrum (early milk produced) in the mother’s milk. Only the newborn can absorb antibodies through the intestines.

Passive and artificial immunity-one receives a shot of pre-made γ globulins. The antibody is harvested from the hyperimmune (convalescent) serum of an animal or individual who has produced this antibody in response to an antigenic challenge.

Humoral Immune Response-immunity associated with antibodies in circulation. These antibodies are produced by B cells that have differentiated into plasma cells.

Burnets clonal selection theory of antibody formation-the appropriate B lymphocyte (cell) in circulation will be selected as it recognizes a specific antigen. The antigen is presented to the B cell on the surface of a macrophage that has ingested it. Once this recognition has occurred, this B cell will activate and start dividing. BCGF (B cell growth factor) is secreted by TH (helper) lymphocytes to aid this process. BCGF is a type of lymphokine (cytokine). After many clones have been made, these B cells will differentiate into plasma cells. BCDF (B cell differentiation factor) is also secreted by TH cells to aid this process. BCDF is also a lymphokine. The plasma cells make the appropriate antibodies to bind with the specific antigen. These antibodies all have the correct combining sites. Some of the B cell clones remain in circulation as BM (memory) cells to respond to this antigen in the future. This is what happens following a natural challenge or an artificial challenge (vaccine). → Anamnestic (memory) response.

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