The Immune System In this part of the course we will study infectious disease. First we will study the immune system, then we will study specific categories of infectious disease. We will begin by studying the different components of the immune system. The next lecture will deal with the function of these components. 1

Understanding The Immune System Risk of Developing an Infectious Illness = Virulence of microorganism X Transmissibility of microorganism Resistance of the potential host When understanding infectious disease, it is important to understand that infection is a universal aspect of life. At this very moment you are infected with many microorganisms. While the term “infection” is usually associated with being ill, you are actually infected all the time. Whether of not the infection causes illness depends on the interaction of the organism and the body. The formula on this slide indicates the factors that determine whether an infectious microorganism will actually produce disease. It states that the risk of developing disease is dependent of three factors. The factors in the numerator are known as the microbial factors. These factors are the virulence transmissibility of the organism. Virulence is the disease producing capability of the microorganism. In other words, does the microorganism produce some product or have some affect of the body that is likely to be pathogenic. Transmissibility refers to the ability of the microorganism to move from individual to individual and its ability to move once it is inside the body. The denominator of this equation contains the resistance of the potential host. That is the ability of the host to resist the affects of the microorganism. We will begin our study of infectious disease by looking at this portion of the equation. We will be looking at the protective factors that are in the body that allow us to respond to most infections to keep us from becoming ill.

Understanding The Immune System RESISTANCE I. Mechanical A. Skin B. Mucous Membranes II. Bio Chemical Defenses A. Generic Immunity B. Specific Immunity III. Acquired Immunity A. Actively Acquired Immunity B. Artificially Acquired Immunity C. Passive Immunity There are many different mechanisms by which the body defends itself from infection. Some of these are mechanical such as the skin and mucous membranes which we will talk about in more detail. There are also biochemical defenses. These defenses are both generic, meaning that they protect us from classes of microorganisms, and specific immunity which protects us from specific microorganisms. Specific immunity can be acquired in a number of different ways. It can be actively acquired, which means that it is acquired by being infected by the microorganism, it can be artificially acquired by immunization, and it can be passively acquired. Essential we can borrow immunity for a short time, for example, the immunity that a child has from its mother at birth. 3

This slide graphically details the different aspects of the body’s defenses Let’s begin by briefly talking about mechanical defenses. Mechanical defenses are physical features of the body that help protect us against disease. The skin is an excellent protector that defends us from disease in many different ways. It is generally dry which provides for a poor growth environment from microorganisms. Second of all, the skin is difficult for microorganism to penetrate. Thirdly, the skin cleanses itself by constantly sloughing off old cells and replacing them with new ones. The respiratory system is the internal system that is most exposed to the external environment. It is not surprising that most diseases that people experience are respiratory in nature due to the large amount air we circulate daily giving us constant exposure to external factors. Both mucous membranes and ciliated cells serve to protect us against these factors. The respiratory system is lined with mucous producing cells that generate a constant fluid flow. The fluid is constantly moving due to the cilia which move the mucous to the back of the pharynx where it is swallowed. When the mucous enters the stomach it encounters an environment that is very hostile to microorganisms. Therefore, the respiratory system, for a large part, is self-cleansing. Other mechanical defenses include earwax and tears.

We move now to the biochemical defenses We move now to the biochemical defenses. The box highlighted is labeled natural immunity, but I think that a better term would be generic immunity. These are chemicals produced by the body that generally defend us against certain classifications of microorganisms. One example would be an interferon. This biochemical product is a general defense against viral infections. Our main focus, however, will be on the other boxes.

Passive immunity refers to immunity that we have borrowed Passive immunity refers to immunity that we have borrowed. One example of this is a newborn infant. One of the last systems to develop in a infant is the immune system which explains why many infectious organisms that are of little consequence to the adult can have catastrophic affects in the newborn. To help defend the newborn until his or her immune system is able to work effectively, maternal antibodies are transferred to the newborn. Maternal antibodies eventually wear out because the baby is not producing the antibody for him or herself. Breast milk also contains antibodies that help protect the infant. In medicine there are also applications of passive immunity. For example, a person can receive antibodies artificially. This is done in cases where people are exposed to rabies or sometimes tetanus. These antibodies are given immediately upon exposure. This is not the same as vaccination because it does not cause the body to produce an immune response. They simply fight the organism until they wear out.

Our focus in this section is acquired immunity Our focus in this section is acquired immunity. There are two cases of acquired immunity. One is actively acquired immunity and the other is artificially acquired immunity. Acquired immunity is specific immunity to microorganisms. An example of actively acquired immunity would be chicken pox which you can only get once. Once you have had the disease, the virus will not longer produce disease in your body because your immune system recognizes it and destroys it.

Artificially acquired immunity is immunity you have from diseases like measles, mumps and tetanus as an outcome of your having been immunized. In this case, antigens that mimic the disease-causing organisms stimulate your immune system to produce antibodies. In some cases, they are simply modified versions of the disease itself. This category of acquired immunity will be the focus of our study on the immune system.