Acquired Immunity?

Can someone please explain acquired immunity with B cells and T cells and their differentiation and how they kill various antigens in the body?

2 Answers

  • 1 decade ago
    Best Answer

    The immune system does not kill antigens, it "recognizes" antigens. The antigen (immunogen is actually a more accurate term) is the molecular part of the invading organism that is recognized by the immune system.

    Acquired immunity is the body's most important protection against invading organisms. But it is very complicated, and really needs much of an entire course to be explained, but I'll try in a few paragraphs.

    The bone marrow is where B and T cells come from. They leave the bone marrow as immature B and T Cells, which have a unique genetic characteristic. As our immune system develops, B and T cell precursors reproduce, and can rearrange the DNA of their receptors as they proliferate in lymph nodes and the thymus respectively. Basically, B and T cells rearrange the genes for their receptors (which are proteins) so that there is a separate clone of B and T cells to recognize ANY POSSIBLE ANTIGEN ON EARTH. This is a completely random process.

    These B and T cells now come together in the lymph nodes, and wait for exposure to the antigen that they recognize. The first time a body ever encounters an antigen, it must be presented to the immune system to activate the B and T cells that recognize that specific antigen. Remember that there are B and T cells for EVERY POSSIBLE ANTIGEN OUT THERE. This exposure does 2 things, first it activates the cells, and second, it generates memory cells so that B and T cells are on standby for future exposures.

    There are 2 types of T cells, CD4+, and CD8+ cells. The CD4+ is the leader here, controlling the whole immune response. CD4+ T cells must have antigens "presented" to them to become activated. The presentation of antigen to T cells (by specialized antigen presenting cells with surface molecules called MHC 2, including B cells and dendritic cells), decides the type of activated T cell that is needed, and thus the type of immune response that is needed: HUMORAL or CELL MEDIATED.

    Generally, extracellular bacteria, fungi, and parasites require HUMORAL immunity, which basically fights organisms OUTSIDE of cells. Viruses (which infect our body by going INSIDE cells) and intracellular bacteria require CELL MEDIATED immunity. Cell mediated immunity means that when a cell becomes infected, the immune system must sacrifice it, and destroy it before infection spreads.

    HUMORAL IMMUNITY: When humoral immunity is needed (for organisms that live outside of our cells), the immune system uses B Cells to eliminate the invading organism. In this case, the B cell with a receptor specific for the antigen recognizes, and presents this ingested antigen from an EXTRACELLULAR ORGANISM to T cells, and their interaction activates the humoral response. This interaction generates CD4+ cells called Th0 cells. The Th0 cells become further specialized, as Th2 cells.

    Now you have Th2 cells with a receptor that recognizes this organism, and a B cell with a receptor that recognizes this organism. What this B cell receptor actually is, is an antibody called IgM. The Th2 cell can then stimulate the B cell to make different kinds of antibodies, and secrete these antibodies into the blood to help eliminate infection.

    Important to note here, when these B and T cells are activated, this is where memory is generated. Memory B cells produce circulating antibodies, but are also on standby waiting to proliferate in case the organism is encountered, and more antibodies are needed. Memory T cells are also on standby. So now next time this organism that requires humoral immunity is encountered by Th0 cells, they will quickly become activated Th2 cells, which will activate B cells (and direct them to become plasma cells), and secrete chemicals called "cytokines", that induce these plasma cells to make the type of antibody needed for the immune response. Plasma cells are basically B cells that become antibody factories.

    Antibodies in circulation have several functions. There are four major types of antibody, IgA, IgG, IgM, and IgE. IgA is secreted into the blood, and into mucous membranes like the respiratory system and GI tracts. The IgA molecule waits in the mucus and when the antigen it recognizes enters the mucus, it coats it and prevents it from penetrating the surface into the body. The IgG molecule is the most important. In the body, it recognizes the antigen, and sticks to the surface. This binding to the surface is recognized by other cells that will destroy the invading organism in a process called opsonization. IgM is the 3rd type of antibody, which also binds to the surface of the organism, and attracts circulating proteins called complement, which can make holes in the organisms membrane and destroy it. The last type of antibody is IgE, which is important in destruction of parasites (and unfortunately as a side effect, is important in allergies like asthma).

    So in summary of humoral immunity, first exposure of an extracellular organism makes memory B cells and T cells that circulate, waiting for the next exposure of this antigen These memory B cells also lead to circulating antibodies. When this antigen is encountered, the memory B and T cells proliferate, and the memory B cell can secrete even more antibody against the organism. The memory T cell recognizes the antigen, and goes from a Th0 cell to a Th2 cell. The Th2 cell secretes cytokines that directs the B cell to become a plasma cell that makes a specific type of antibody needed. This is the basis of vaccination, so that there is circulating antibody against an organism, and memory B and T cells that are on standby).

    CELL MEDIATED IMMUNITY: when an organism like a virus (or an intracellular bacteria like tuberculosis) lives inside of one of our cells, the immune response is different. I'll use liver cells with hepatitis as an example, but this is the same idea for all cells that become infected. When a liver cell becomes infected with hepatitis, CD4+ cells called Th0 cells become Th1 cells, which direct the immune system to kill the infected cell.

    Th1 cells do this by secreting cytokines that do 2 things. First, they activate macrophages, which can become powerful enough to ingest an infected cell. Also, the cytokines activate T cells called CD8+ cells, which induce a cell to kill itself. Infected liver cells will present to CD8+ cells the antigen of the virus using a molecule called MHC 1 (compared with MHC 2 as above). This stimulates CD8+ cells to make the infected liver cells commit suicide, taking the virus inside down with it.

    As mentioned above, the same idea with memory exists here, there are memory Th0 cells, and memory CD8+ cells waiting for future infections.

    So in summary, here are the most important points:

    1. Many populations of B and T cells exist, where there is one population of B and T cells for every possible antigen out there.

    2. The first time an antigen is encountered, the population of B and T cells that recognize the specific antigen are activated. T cells require that the antigen be presented to them, and this allows the T cell to run the show. This activation creates effector cells to clear the first infections, and memory cells, which wait for the next antigen exposure, and produce circulating antibody for protection.

    3. The next exposure, the memory cells can skip a lot of the process. When an extracellular organism (most bacteria) is presented to a T cell (only by specific antigen presenting cells), the Th2 route drives the humoral response, and antibody is the major player.

    4. When intracellular organisms are encountered, Th0 cells follow the Th1 route, which activates macrophages and CD8+ cells. Infected cells present antigens to signal to CD8+ cells that they are infected, and the CD8+ cells induce cell suicide.

    5. Extracellular Organism = Humoral Immunity = Antigen presenting cell with MHC2 activates a Th0 --> Th2 . . . Th2 modulate the response of B cells to create the proper form of antibody.

    Intracellular Organism = Cell Mediated Immunity = Antigen presenting cell presents antigen with MHC 2 to Th0 cell --> Th1 cell. Th1 cell activates macrophages and CD8+ cells. Any infected cell presents with MHC 1 to CD8+ cells. CD8+ cells induce suicide of infected cell, and intracellular organism dies with it.

    It's a lot to follow, but try to draw the steps out sequentially in your head.

  • 4 years ago

    b. dependent exclusively on cell-mediated responses

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