Molecular Biology of the Cell - Volume 3 - Alberts B., Bray D., Lewis J., Raff M., Roberts K., Watson J. 1994

From Cells to Multicellular Organisms
The Immune System

Our immune system saves us from certain death due to infectious diseases. Any child born with a severely impaired immune function is doomed to rapid death unless extraordinary measures are taken to isolate them from a multitude of infectious agents, including Bacteria, Viruses, pathogenic Fungi, and parasites. Not only humans, but any vertebrate with immunological deficiency (immunodeficiency) faces the threat of death.

All vertebrates possess an immune system. In invertebrates, defense systems are more primitive, typically based on phagocytic Cells. Such cells—primarily macrophages and neutrophils—also play a vital role in defending against infection in vertebrates (Section 17.5.1), yet they represent only a part of a much more complex and sophisticated defense system.

Immunology, the science of The Immune System, grew out of a simple observation: people who recover from certain infectious diseases become refractory (“immune”) to them, meaning they rarely fall ill with them again. This Immunity is highly specific: someone who has had measles is protected against measles viruses, but not against other common viruses such as those causing mumps or chickenpox. This kind of Specificity is a fundamental feature of all immune responses.

Many immune system responses lead to the destruction and clearance of invading parasitic organisms and the toxic molecules they produce. Because such immune reactions are aimed at destruction, it is crucial that they are triggered only by molecules foreign to the Organism, rather than its own. The ability to distinguish non-self from self is the second fundamental property of the immune system. Occasionally, it mistakes “self” for “non-self” and initiates destructive actions against the organism's own molecules. Such autoimmune diseases can be fatal.

The immune system evolved in vertebrates as a means of defense against infection by microorganisms and larger parasites; however, much of what is known about immunity has been learned by studying the responses of laboratory animals to the administration of non-infectious agents, such as foreign Proteins and Polysaccharides. Almost any macromolecule foreign to the recipient's body can elicit an Immune Response. A substance capable of eliciting an immune response is called an antigen (i.e., antibody generator). Remarkably, the immune system can distinguish even very similar Antigens, such as two proteins differing by only a single amino acid, or two optical isomers.

There are two main TYPES OF IMMUNE responses: 1) humoral responses and 2) Cell-mediated immune responses. Humoral responses involve The production of Antibodies—proteins also known as IMMUNOGLOBULINS. Antibodies circulate in the Blood and permeate other Body Fluids, where they specifically bind to the foreign antigens that induced their synthesis. Binding by antibodies neutralizes viruses and Bacterial toxins (such as tetanus or botulinum toxin) by blocking their ability to attach to receptors on target cells. Bound antibodies serve as “tags” for microorganisms destined for destruction, facilitating their uptake by phagocytes or activating a specialized system of blood proteins (called Complement) that kills these microorganisms.

Cell-mediated immunity—the second type of immune reaction—involves the generation of specialized cells that react with a foreign antigen On the surface of the organism's own cells. The reactive cell can kill a host cell infected with a virus that displays viral proteins on its surface, thereby destroying the infected cell before viral Replication is complete. In other cases, The Cell's response consists of secreting chemical signals that stimulate macrophages to destroy the invading microorganisms.

The primary challenge facing immunology has been to understand how the immune system specifically recognizes and aggressively reacts to a virtually limitless array of foreign molecules while refraining from attacking the tens of thousands of different macromolecules produced by the organism's own cells. To approach the answer to this question, we will first examine the cells responsible for the two Types of Immunity. Next, we will systematically review the function and Structure of antibodies, The Complement System, and the Specific features of cellular immunity.