Bio 113- Immunity
Cell types - As we have seen, here are many different types of cells in the Immune System. Some, known as Macrophages and Natural Killer Cells, are involved in complex relationships in which they recognize B-Cell and T-Celland then attack non-self cells and substances, These "Killer" cells then process the non-self material for detection by other elements of the immune system. Other are involved in an intimate selective response to foreign invasion.

star Lymphocytes become B or T cells (Bio 113)

All the components of the immune system are tied together much like the members of an army. They use molecular shape and chemical signals to coordinate their defense against invaders. The entire Immune response is exceedingly complex (almost complicated) with many cell types interacting in variable manners.

T - Cells and B -Cells: Lymphocytes originate from the bone marrow and become specialized as they are released into the blood stream. About half of them are carried to the thymus where they mature into T-cells. There are several different types of T-cells. Each has a unique role in the immune process. T-cells have protein receptors on their cell surfaces that specifically bind to foreign cells and substances. They also have other receptors on their surfaces that identify other cells within the body with which they are designed to interact.

T-cells come in three basic varieties:
  • Cytotoxic T-cells, which attack Invaders with chemicals;
  • Suppressors T-cells, which regulate the attack
  • Helper T-cells, which urge the other immune cells, e.g. B-cells, into action.

When blood or tissue is invaded, the various phagocytes, monocytes and neutrophils begin consuming the invaders - any type, dead or alive. The T-cells attack after developing an affinity for certain molecular shapes of the enemy. The T-helper cells can also activate B lymphocytes, or B-cells, which make antibodies.

When Invader antigens enter the body, Macrophages take up some of the antigen and attach it to class MHCII molecules. The MHC molecules present the antigen (the antigen is a little piece of the invader) to T-helper cells and the T-helper cells bind the presented antigen, which stimulates the T-helper cells to divide and secrete molecules called cytokines (for cell division). The cytokines activate any B-cells that have also bound the antigen and the activated B-cells divide and secrete antibodies. The secreted antibodies bind the antigen and mark it for destruction.

B-cells are lymphocytes which are not processed in the thymus. The B-Cells pass through lymphoid tissue, where they are processed before they move on to the lymph glands. Mature B cells can have thousands of antibodies on their surface. Each of these antibodies possesses an binding site for antigen. Each particular B lymphocyte is also specific for only one (or a small number) antigen. Some of the useful B-cells, stimulated by the T-cells, turn into "memory" cells. The next time the invader appears, the body will be prepared and can mount an instant and powerful antibody defense.

Antibodies (immunologlobulins) are not another type of cell, but are protein molecules that are designed to stick to, disrupt and disable the invaders until they are gobbled up or absorbed. Antibodies stick the tiny invaders into clumps which are easy targets for the phagocytes to gobble up.

Selected B-Cells produce Antibodies (Bio 113)

The antibodies circulate through the blood and lymph system and will bind to the invaders. When blood is allowed to sit quietly, the red blood) and white cells settle or to the bottom of the tube, leaving behind a straw-colored liquid called plasma. The antibodies, and other soluble components of the blood, are located in the plasma. One can often make money selling "plasma" and its proteins.

In addition to antibodies, B lymphocytes possess other characteristic surface proteins: Major Histocompatability Complex (MHC) proteins. There are two classes of MHC molecules; MHCI and MHCII. One difference between the two types of MHC molecules is that all the cells in the body have MHC class I protein on the surface.

As you remember, the MHC proteins function in the identification of self. However, MHC-Class II are proteins only expressed by cells of the Immune system. A second function for these MHC proteins is to present an antigen (below) to T helper cells to activate an immune response which will stimulate both B-cells (which make antibodies) and other T-Cells (which can eat or kill the invader).

Tc Cells bind to the CD 8 proteinMHC type I proteins are found in the plasma membrane of all the cells of your body. MHC class I molecules bind fragments derived from the invader within the cell. Pieces of the invade are transported and displayed on the surface of the infected cell for recognition by cytotoxic (Tc) lymphocytes.

Differential structural properties of MHC class I and class II molecules account for their respective roles in activating different populations of T lymphocytes. Tc lymphocytes bind peptides presented by MHC class I molecules.

T - Helper (TH) lymphocytes are the cells that bind antigenic peptides presented by MHC class II molecules. MHC class I and class II molecules differentially bind CD8 and CD4 receptor molecules on the T- Cells. MHC class I molecules specifically bind CD8 molecules expressed on cytotoxic TC lymphocytes. MHC class II molecules specifically bind CD4 molecules expressed on helper TH lymphocytes.

MHC type II next