The immune system is designed to defend the body from attack or invasion by foreign substances. This is accomplished through a complex system of proteins and cells, including complement proteins, antibodies and white blood cells, each with a specialized function. Normally, complement protein, together with antibodies and white blood cells, act beneficially to protect the body by removing germs, foreign cells, and immune complexes.

Complement Causes Disease Any number of harmful stimuli can cause complement activation. In the case of paroxysmal nocturnal hemoglobinuria, complement activation leads to red blood cell destruction, severe pain, and blood clots. In the case of cardiopulmonary bypass surgery or myocardial infarction, complement activation leads to significant heart damage.

T-Cells Cause Disease T-cells are specialized white blood cells that play a critical role in normal immune responses. T-cells recognize proteins, or markers, on the surface of cells as foreign (foreign markers are also known as antigens) and thereby initiate an immune response. This response focuses the attack on the antigen-bearing tissue and further directing the production of antibodies against the antigens. All of which leads to the elimination of the antigen-bearing cell or tissue. As T-cells mature in their early development, most acquire the ability to identify foreign surface markers ("non-self") as suitable for attack, while differentiating these from markers on normal body tissues ("self"). The occasional "rogue" T-cells that are capable of reacting to self markers on healthy tissues are normally destroyed during maturation. This destruction occurs via a process of cell suicide, termed apoptosis.

T-cells travel through the bloodstream as well as a specialize tissue called lymphoid tissue. Lymphoid tissue contains such organs as the spleen and lymph nodes. Within these tissues, T-cells are likely to encounter another cell type called dendritic cells (see below) whose function is to capture and then present circulating foreign markers to T-cells. Once a T-cell recognizes its particular foreign protein target, it initiates a multi-faceted immune response against that target. The activated T-cell recruits additional T-cells, as well as inflammatory cells such as macrophages, which together help eliminate the source of the foreign marker. In addition, activated T-cells also recruit other lymphocytes, called B cells, to make antibodies that bind to the target protein; this process also helps eliminate the invading organism. Finally, still other T-cells are triggered to "suppress" the immune response once the target has been destroyed.

T-Cells in Autoimmune Disease Unfortunately, in autoimmune disease, dysfunction and dysregulation of the immune system allows for rogue T-cells to survive so that they can attack the patient's own tissues. Some diseases associated with this condition include multiple sclerosis , diabetes mellitus, rheumatoid arthritis, and psoriasis.

Dendritic Cells Dendritic cells (DCs) have recently come to be appreciated as critical controllers of the immune system. In order for an immune response to occur, foreign proteins or peptides (for instance from a bacterial or viral infection) must be displayed by a specialized cell whose function it is to present these markers to T-cells. These specialized cells are also known as antigen-presenting cells. While dendritic cells are an extremely rare immune cell type, they are the most potent of all the antigen presenting cells. DCs capture antigens in tissues distant from the central lymphoid tissues also known as the periphery. DCs process and display the antigen fragments on their cell surface, and then migrate from the periphery to the T-cell areas of the lymphoid organs. Once back in the lymphoid organs, DCs attract resting T-cells and present their antigen load, thus activating the T-cells to begin an immune response.