Immunology

In the late 18th century Dr. Edward Jenner stumbled upon the notion of the immune system following his discovery that cowpox protected him against a human pathogen known as smallpox. This process of immunity by introduction of more mild strains of a disease became known as vaccination. Unfortunately, very little was understood about the etiology of infection and the subsequent induction of the host immune response. Dr. Robert Koch, over 100 years later, established the first relationship between cause and effect as it relates to infection. Out of his work came specific postulates used to understand which pathogens are fundamentally responsible for specific diseases and what components of those pathogens contribute to disease progression. The concept of immunology was born.

Immunology is the study of the immune system, the primary defense mechanism of the body. In humans, the immune system is composed of specific immune cells and produces and utilizes numerous unique, chemical signals. Each component of the immune system works collectively to prevent invasion by foreign substances, or pathogens. Examples of such pathogens include bacteria, viruses, fungi, and parasites. Each utilizes a specific method of transmission such as fecal-oral, airborne, direct inoculation, direct contact, and congenital. To elicit an immune response, these pathogens must not only be foreign to the body, but they must be large enough molecules, chemically complex, and degradable. Still heavily studied area, the human immune system consists of both innate and acquired responses to pathogenic invasion with both types of responses interacting on many complex, biological levels.

Typical components of the innate immune system include normal physiologic and chemical barriers such as the skin, saliva, the pH of the stomach, tears, enzymes, and mucous. During the innate response, the human body utilizes immune cells such as neutrophils, monocytes, NK cells, and tissue macrophages. These cells respond to specific foreign patterns through preexisting pattern recognition receptors. Each type of cell has the ability to kill the invading organism through a nonspecific process known as phagocytosis, a mechanism first discovered by the Russian immunologist Dr. Elie Metchnikoff. This process includes engulfment of the pathogen and fusion with acidic enzymes within the host cell. Innate immune cells can also kill through the complement cascade as well as extracellular mechanisms involving soluble mediators. Complement activation includes first lysing the pathogens, and then coating them with a material targeting them for phagocytosis. Complement involves the following three pathways: classical, alternative, and lectin binding. All converge on a common pathway where a pore is created within the target cell membrane leading to lysis of that cell. In innate immunity, the characteristic immune response includes chemical messages in the form of cytokines, pyrogens (fever producers), interferon, and complement. While innate immunity is an effective means of fighting off foreign bodies, it is sometimes not enough. Thankfully, our bodies possess another type of immunity known as acquired immunity.

Acquired, or adaptive immunity, can be divided into humoral and cell mediated immunity. The response time is much slower when compared with innate immunity. Both humoral and cell-mediated responses [Page 885]depend on specific cells known as B and T cells which develop and mature in the primary lymphoid organs of the bone marrow and thymus gland, respectively. Following maturation, these cells leave the primary lymphoid organs and move to secondary lymphoid organs, which include the spleen, lymph nodes, and tonsils. B cells recognize intact pathogen, whereas T cells only recognize pathogen that has been broken down into protein components and presented on the cell surface of another immune cell. B cells communicate with other cells through a specific receptor, whereas T cells must bind using a receptor and an additional cell surface molecule known as the MHC. There are two important types of T cells, CD8 and CD4, and each binds to a specific set of cell surface molecules.

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