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The immune system, composed of antigens, antibodies, and several specialized leukocyte (white blood cell) types, offers the human body adaptable protection against infections and invasion by foreign molecules and cells. Besides responses by antibodies and cells, the immune system is also composed of a complement-activated pathway that allows an alternative to respond to infectious organisms.

Although able to attack these pathogens, the immune system is also able to recognize the host human cells and molecules and can selectively not respond to “self.” Uncontrolled immune responses can lead to tissue damage or death.


The human immune system is responsible for generating a protective response to infective organisms (e.g., bacteria, viruses, and parasites) and foreign cells (e.g., tumor and transplant) while both recognizing the host body and limiting damage to itself (self–nonself theory). As attacks are always evolving, the immune system must be capable of adapting to each new challenge. The immune system also provides an inflammatory response to trauma. When tissue or cells are damaged, molecules that act as endogenous danger signals can be released. These danger signals may include molecules such as uric acid (produced by purine metabolism, Chapter 4), which, if present at high concentrations, can form crystals that innate immune sensors recognize and thus activate immune and inflammatory responses.

Innate immunity refers to preformed host defense systems that can provide immediate host defense activities without first being trained to distinguish self from invader. Elements of the innate immune system often contain structural recognition motifs that allow them to identify likely pathogens to target. Molecules that are found in microbes without structural homologs in human cells, such as flagellin or unmethylated deoxyribonucleic acid (DNA), are other examples of danger signals that induce innate immune activation when their structure is recognized by innate immune sensors.

The adaptive immune system is capable of generating a response very specific to the structure of an invading organism or molecule (antigen, see below) as well as the ability to remember that structure via memory cells (immunological memory). The repeat activation of the immune system by an antigen that has previously induced an immune response is normally much quicker and stronger. The invading pathogen serves as a target, which elicits a response specific for that antigen from immune cells. This response may include the production of a specific antibody against that pathogen (humoral immune system) or the activation of immune cells that either attack and kill the offending organism or orchestrate this activity, the cell-mediated immune system. These cell types include lymphocytes (including T and B lymphocyte types), monocytes, macrophages, dendritic cells (DCs), neutrophils, eosinophils, and basophils.

Passive immunity involves antibody molecules that are transferred to the baby from the mother’s active immune system through the placenta. Immunoglobulin (Ig) G (see below) is the single ...

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