Question

Compare the responses by serum complement (alternative pathway; 3 responses) to antibody- mediated responses (5 responses) to bacteria in the bloodstream. What host cells are involved in each response, and how are bacteria or other antigens destroyed or effectively removed?

Answer 1

The complement system is a part of innate immune system which enhances/complements the ability of antibodies and phagocytic cells to clear pathogens and damaged cells from an organism. It consists of number of small proteins synthesized in liver and which circulate as inactive precursors in blood. Following 3 responses are seen in serum complement system.

• After stimulation by several triggers proteases in the system cleave specific complement proteins to release cytokines and initiates the amplyfying cascade and cytokines are released.
• In alternative pathway the complement protein C3 is converted by C3 convertase enzyme to C5 convertase (C3BbC3bP) enzyme. This enzyme then cleaves C5 to C5a which is a potent anaphylatoxin and C5b.
• C5b then recruits and assembles C6, C7, C8 and multiple C9 molecules to attack membrane of pathogen creating a hole or pore in the membrane to kill the pathogen.

In antibody mediated response antibodies are secreted by B cells. Antibodies attach to the bacteria and help in their destruction. Following 5 responses are seen in antibody mediated system,

• B cell expresses B cell receptors (BCR) which have antibodies specific for antigen detection. B cell is activated when it encounters antigen which is then endocytosed, processed inside and presented on B cell surface along with MHC II proteins.
• Activated B cells bind to TH cells/ helper T cells. This interaction triggers release of cytokines by TH cells in turn inducing B cell division and forming plasma cells or memory cells.
• The memory cells remain inactive till they encounter same antigen. On activation memory cells are converted into plasma cells producing large number of bodies.
• The antibodies will bind antigens which will interfere the interaction between host and foreign cells.
• The antigen-antibody complexes are then attacked by macrophages or killer cells and phagocytosed.