Question

1. What are 3 ways by which bacteria can overcome, evade or avoid phagocytosis? 3pts

2. Some bacteria can stop the activation of complement. Suggest 2 things bacteria might do to complement proteins to stop or prevent complement activation. Describe how stopping complement activation would protect the bacteria. 4pts

3. Explain how CD8, CD4 and B cells work together either in the lymphoid tissue OR at the site of infection.

Answer 1

1. There are different ways in bacteria resist phagocytosis by the host cells:

A) Avoid bacterial contact with phagocytes

• Regional isolation - Bacteria enter into a place that is inaccessible to phagocytes such as inside liver, brain, surface tissues, etc.
• No inflammation - As the phagocyte recruitment occurs with inflammation, so it avoids inflammatory substances.
• Blocking chemotaxis activity - Some bacteria are capable to kill phagocytes cells. Eg. Streptococcal streptolysin on neutrophils, Mycobacterium tuberculosis on Leukocytes, etc.
• Making of antigen surface - As the immune response occurs only when the host cell recognise foreign antigen. It is done by coating host cell surface antigen on its surface so recognized as self itself. Eg. Staphylococcus aureus through cell-bound coagulate and clumping factor.

B) Prevention of phagocytes engulfing

Surface components prevent phagocytosis such as :

• Polysaccharide capsule
• M protein and fimbriae
• Surface slime
• O polysaccharide of gram-negative LPS
• K antigen - The capsular antigen which is acidic in nature.
• Protein A - Which bind with the Fc receptor of IgG thus prevent the cell-binding domain of antibody, thus unable to opsonic factor release is inhibited. Thus prevents killing of bacteria.

C) Survival inside phagocytic cells.

The bacteria have invasions that promote the bacteria to enter into the phagocytic cell. Then the survival mechanism various, that is:

• Prevent phagolysosome formation - The bacteria will only be lysed with the release of the lysosomal secretions, which is released into phagosome by the formation of phagolysosome as the bacteria is engulfed by the phagocytes by formation of vacuole inside which is the bacteria and that structure is the phagosome. The lysosomal secretion will kill bacteria so they prevent the fusion of phagosome and lysosome.
• Survival inside phagolysosome - Some bacteria are even capable to resist the lysosomal secretion due to presence of surface components. Such as Mycobacterium has waxy, hydrophobic cell wall with mycolic acid, Capsular component of gram-negative bacteria, etc.
• Escape from phagosome - In some bacteria, presence of phospholipase A help escape of Rickettsia while phospholipase C help Listeria