Question

The adaptive immune system has two major effector arms: humoral and cell-mediated. Discuss the need for such separate systems from the standpoint of common pathogenic organisms and substances. What might you think are the "weak links" in these systems?

1. How have some disease-producing organisms exploited these?

2. Does this help explain why it is extremely difficult to generate effective vaccines for some organisms? Give an example and explain your reasoning.

Answer 1

Depending on the kind of foreign invasion, two different immune responses occur:

The humoral response (or antibody‐mediated response) involves B cells that recognize antigens or pathogens that are circulating in the lymph or blood. The response follows this chain of events:

1. Antigens bind to B cells.
2. Interleukins or helper T cells costimulate B cells. In most cases, both an antigen and a costimulator are required to activate a B cell and initiate B cell proliferation.
3. B cells proliferate and produce plasma cells. The plasma cells bear antibodies with the identical antigen specificity as the antigen receptors of the activated B cells. The antibodies are released and circulate through the body, binding to antigens.
4. B cells  produce memory cells. Memory cells provide future immunity.

• The cell‐mediated response involves mostly T cells and responds to any cell that displays aberrant MHC markers, including cells invaded by pathogens, tumor cells, or transplanted cells. The following chain of events describes this immune response:

1. Self cells or APCs displaying foreign antigens bind to T cells.
2. Interleukins (secreted by APCs or helper T cells) costimulate activation of T cells.
3. If MHC‐I and endogenous antigens are displayed on the plasma membrane, T cells proliferate, producing cytotoxic T cells. Cytotoxic T cells destroy cells displaying the antigens.
4. If MHC‐II and exogenous antigens are displayed on the plasma membrane, T cells proliferate, producing helper T cells. Helper T cells release interleukins (and other cytokines), which stimulate B cells to produce antibodies that bind to the antigens and stimulate nonspecific agents (NK and macrophages) to destroy the antigens.

The weaklinks in these 2 systems can be said as:

1) The humoral immunity protects against extracellular pathogens and The cell mediated immunity protects against intracellular pathogens.

2) The onset of cell mediated immunity is delayed.

Since these two systems do their work in a remarkably successful manner there are not much flaws to be pointed out.

Vaccines are among the most cost-effective ways to protect against infectious diseases. A well known example is the large drop in vaccination with the measles, mumps and rubella (MMR) vaccine.