Evasion of Host Immune System

Question 1

How does bacteria evade complement?

Answer 1

Gram+ cells have a thick peptidoglycan layer; Others have a capsule. Hold MAC away with long O-Ag side chains. Incorporate sialic acid into surface molecules to prevent C3b binding. Or have enzymes that degrade C3b; LPS residues that inhibit complement binding.

Question 2

How does bacteria evade the immune system?

Answer 2

Evade complement, Inhibit chemotaxis or phagocytosis, and destroy phagocytes.

Question 3

How do bacteria inhibit chemotaxis?

Answer 3

Cord factor of Mycobacterium tuberculosis inhibits neutrophil migration; Elastases inactivate C5a. C5a peptidase of streptococcus degrades C5a.

Question 4

How do bacteria inhibit phagocytosis?

Answer 4

The bacteria’s capsule is usually a poorly immunogenic polysaccharide; Causes T-independent responses. Porins can inhibit phagocytosis; Protein M in Strep pyogenes blocks binding of C3b. Can hide in clot or granuloma.

Question 5

Describe T-independent responses and T-dependent responses. Which is better?

Answer 5

T-independent responses do not involve T cells; T-dependent responses are better. T-dependent responses require proteins.

Question 6

How does bacteria survive in phagocytic cells?

Answer 6

Failure to stimulate cellular killing mechanisms. Resistance to lysosomal enzymes and granules, block oxidative burst, resist ROS w/ catalase or SOD.

Question 7

Describe antigen variation and how bacteria use it to hide.

Answer 7

Bacteria can change their cell surface antigens in order to avoid antibodies and the adaptive immune system.

Question 8

Describe antigen shedding

Answer 8

Pathogen sheds its cell antigen so that the immune system will attack the antigen rather than the cell; Similar to fighter jets shooting lights to confuse a missile.