Question

Antibodies are produced by B cells, lymphocytes that are created and mature in red bone marrow. The function of these cells is to migrate to lymphoid tissue such as the spleen or lymph nodes and secrete protective antibodies in response to infection. When an antigen-presenting cell, like a B cell, makes contact with an antigen that can combine with its particular receptor, the APC and antigen bind. The APC then internalizes the antigen and digests it. Next, the APC displays digested antigen fragments on its surface by combining them with its major histocompatibility complex (MHC). The MHC is a collection of glycoproteins embedded in the plasma membrane. Class I MHC is found on all mammalian nucleated cells. Their presence identifies a cell as "self," preventing the immune system from making antibodies that would be harmful to our own tissues. Class II MHC is found on APCs (B cells, macrophages, and dendritic cells). Displaying antigen fragments bound to its MHC class II molecules attracts the appropriate T helper cell to the B cell. The TH cell makes contact with the fragment presented on the APC, and then the T helper cell produces cytokines that activate a B cell, which divides into a large clone of cells, including antibody-producing plasma cells and long-lived antibody cells. This antigen is known as a T-dependent antigen because it requires a TH cell to activate the B cell. T-independent antigens stimulate B cells directly, without the help of T cells. T-independent antigens tend to be molecules consisting of repeating subunits, such as polysaccharides or lipopolysaccharides. Bacterial capsules are a common example of T-independent antigens. These repeating units can bind to multiple B cell receptors, activating the B cell. These antigens tend to provoke a weaker immune response and do not produce memory cells. Question 6: Place the steps of B cell activation and antibody production in order chronologically for a T-dependent antigen. I - Antigen fragments are displayed on the B cell surface, attracting a matching T helper cell. II - The activated B cell begins producing antibodies. III - Antigen is phagocytized and digested. IV - APC receptors recognize and attach to antigen. V - T helper cell secretes cytokines, activating a B cell.

Antibodies are produced by B cells, lymphocytes that are created and mature in red bone marrow. The function of these cells is to migrate to lymphoid tissue such as the spleen or lymph nodes and secrete protective antibodies in response to infection. When an antigen-presenting cell, like a B cell, makes contact with an antigen that can combine with its particular receptor, the APC and antigen bind. The APC then internalizes the antigen and digests it. Next, the APC displays digested antigen fragments on its surface by combining them with its major histocompatibility complex (MHC). The MHC is a collection of glycoproteins embedded in the plasma membrane. Class I MHC is found on all mammalian nucleated cells. Their presence identifies a cell as "self," preventing the immune system from making antibodies that would be harmful to our own tissues. Class II MHC is found on APCs (B cells, macrophages, and dendritic cells). Displaying antigen fragments bound to its MHC class II molecules attracts the appropriate T helper cell to the B cell. The TH cell makes contact with the fragment presented on the APC, and then the T helper cell produces cytokines that activate a B cell, which divides into a large clone of cells, including antibody-producing plasma cells and long-lived antibody cells. This antigen is known as a T-dependent antigen because it requires a TH cell to activate the B cell. T-independent antigens stimulate B cells directly, without the help of T cells. T-independent antigens tend to be molecules consisting of repeating subunits, such as polysaccharides or lipopolysaccharides. Bacterial capsules are a common example of T-independent antigens. These repeating units can bind to multiple B cell receptors, activating the B cell. These antigens tend to provoke a weaker immune response and do not produce memory cells.

Question 6: Place the steps of B cell activation and antibody production in order chronologically for a T-dependent antigen.

I - Antigen fragments are displayed on the B cell surface, attracting a matching T helper cell.
II - The activated B cell begins producing antibodies.
III - Antigen is phagocytized and digested.
IV - APC receptors recognize and attach to antigen.
V - T helper cell secretes cytokines, activating a B cell.

Added by Martha O.

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