1. Invertebrates recognize foreign tissue by the self-marker protein and by amoeboid phagocytes engulfing any cells that are without the label, i.e., invaders. The disadvantage of this is that phagocytes will not engulf copycat invading cells with a protein marker that resembles the self-marker.
2. The skin, digestive tract, and respiratory tract employ nonspecific defense mechanisms such as bacteriocidal secretions, production of acid and mucus, ciliary motion, and simple barriers to entrance into the body.
3. Macrophages kill cells by engulfing the ones presenting the appropriate antigen. Natural killer cells kill not the invading pathogen, but rather, the infected cell, by punching holes in the cell membrane and allowing the infected cell to lyse.
4. The complement system is composed of a series of about 20 freely-circulating proteins in the blood. When they encounter an infected cell, they assemble an "attack complex" that creates a pore in the membrane of the infected cell. Water can then enter the cell, ultimately causing it to burst.
5. MHC I proteins are found on all nucleated cells in the body, identifying it to the body as "self," so that when an immunological attack is in progress, the body does not destroy its own cells along with the pathological invaders. MHC II proteins are found only on macrophages, B-cells, and CD4+ T cells; they function in identifying each other.
6. Macrophages activate helper T cells by engulfing the invading pathogen and presenting the pathogen's antigens on its surface for the helper T cells to recognize. Once activated, helper T cells secrete lymphokines that mobilize cytotoxic T cells to fight off the pathogen.
7. Antibodies are produced by plasma cells (B lymphocytes). IgM functions to attract and persuade the proteins of the complement system in for attack. IgG marks the invading pathogen for phagocytosis.
8. The basic structure of an antibody molecule is four polypeptide chains: two identical, short light chains, and two identical, long heavy chains held together by disulfide bonds, that form a Y-shape. Specificity resides in the terminal half of each arm of the Y. A T receptor resembles one arm of the antibody molecule, with alpha and beta chains, that has both constant and variable regions.
9. When a B lymphocyte responds to an antigen, it is stimulated to replicate. The B cell replicates madly at that point, and is said to be enlarging its clone of the specific antibody-secreting B cells required to fight the invading pathogen. In this way, a tremendous variety of B lymphocytes can circulate throughout the system in small number, and only the ones that become necessary proliferate.
10. Lymphocytes create immune receptors out of "gene kits." By combining a smaller number of components several different ways, millions of receptors can be produced from only a few hundred copies of different genes.
11. Natural immunological tolerance refers to the lack of an immunological response against one's own tissues. The immunological basis for these processes is the elimination or suppression of those particular clones of lymphocytes. Acquired immunological tolerance refers to the process in which foreign tissue introduced in the embryo is not recognized as nonself and grafts of that tissue are then not rejected by the adult.
12. Trypanosomes elude the immune defenses of their hosts by changing their surface antigens on their coats: an immune response is created to a specific antigen, but then the antigen is changed and the host can no longer recognize the pathogen. AIDS interferes with the immune response by three means: (1) AIDS-infected cells release progeny viruses that infect other CD4+ cells, killing them and releasing more viruses; (2) AIDS viruses cause CD4+ cells to secrete a factor that blocks other T cell responses to the viral antigen; and (3) AIDS viruses block transcription of the MHC genes, hindering the recognition and destruction of infected cells. Usually death results from other infections or cancers, not directly from AIDS.