Streptococcal bacteria - On page 112, Dr. Moalem describes streptococcal bacteria which are responsible for a wide range of human diseases. They exhibit a phenomenon called molecular mimicry in order to trick the immune system. The bacteria are able to mimic heart, joint, and even brain cells in humans. This relates to Big Idea 4 (biological systems interact, and these systems and their interactions possess complex properties) since the bacteria's interaction with cells in the body leads to the immune system producing antibodies in an attempt to rid the body of the invaders.
First define what is molecular mimicry, and does it relate to either of the two types of mimicry we had learned in a previous unit (batesian or mullerian mimicry)? Also describe what type of symbiotic relationship the bacteria has with its host. Then pick one of the type of human cells the bacteria can mimic (heart, joint, or brain) and describe two consequences resulting from the attack of the immune system towards this region of the body.
Finally, describe the type of antibodies our immune systems use in order to combat the streptococcal bacteria and how they fight the bacteria.
Faith Teodoro (fteodor4@students.d125.org)
When an organism such as a pathogen exhibits molecular mimicry, “[it displays] characteristics of human cells in order to trick the immune system” (112). Because of this, the body’s immune system will attack its own cells, thinking that they resemble the pathogen. This is done specifically by a part of one of the pathogen’s protein closely resembling a part of a different protein in the body. For example, the first 10 amino acid sequences could be exactly the same on each a protein from the pathogen and a human. When a T-helper cell recognizes the foreign invader, its receptors bind to the short segment (10 amino acids in this example) of a foreign protein. Macrophages then engulf the pathogen and display its protein fragment (peptide) on their surfaces to presents it to the millions of circulating T cells. But because the presented peptide closely resembles one of the body’s own proteins, the activated immune system will not only attack the foreign invaders but also body cells with the similar protein.
ReplyDeleteThis is similar to Batesian mimicry, but contrasts in the result from the predator (which is analogous to the immune system in this example). Molecular mimicry usually has a harmful pathogen mimicking the harmless body cells, rather than vice versa in Batesian mimicry. Also, the purpose of a pathogen expressing molecular mimicry is to trick the immune system (predator) to attack the body’s harmless cells, rather than protecting the pathogen itself from harm. This characterizes a parasitic relationship between the pathogen and the body. Although molecular mimicry does not prevent the immune system to attack the pathogens, the pathogens gain an advantage in the end by a weakened and tricked immune system which will help them survive and reproduce while destroying the host.
The consequences of this on the body can be dire. An example is the rheumatic fever caused by the streptococcal bacteria leading to heart disease. The immune system’s “antibodies attack the heart valve because the infecting bacteria resembles it in some ways” (112). Common complications are valve stenosis (narrowing of the valve which results in decreased blood flow), valve regurgitation (a leak in the valve which allows blood to flow in the wrong direction), and damage to heart muscle (resulting poor pumping function. More severe complications might even lead to heart failure or heart attack later in life.
The body first attempts to kill the streptococcal bacteria when it enters the mouth. The IgA antibody in saliva reacts with antigens on their surface to repel them. When the bacteria enters the body, proliferating T-cells help B- and plasma cells secrete the IgM antibody specific to the antigen on the bacteria. Which mark them for later destruction by macrophages or white blood cells.
Sources:
http://www.direct-ms.org/molecularmimicry.html
http://www.mayoclinic.com/health/rheumatic-fever/DS00250/DSECTION=complications
http://www.microbiologytext.com/index.php?module=Book&func=displayarticle&art_id=400
(posted by Raymon Cao, racao4@students.d125.org)