Immunology Continued Specific Defenses of the Immune System.

Specific Defenses of the Host Acquired Immunity: protection a person develops over the course of one’s life. –2 Types: Passive: antibodies are transferred from one person to another. In other words, one person acquired the disease or was vaccinated against the disease and the antibodies their body developed were transferred to another person. An example would be antibodies that an infant acquires through breast feeding. This kind of immunity lasts only as long as a continual source of the antibodies are present or as long as the antibodies themselves are present in the recipient.

Active: An individual is exposed to microorganisms or foreign substances and the immune system responds by producing immune cells. This immunity is generally longer lasting and provides future protection against reinfection. These two types of immunity can be broken down further into the following categories: –Naturally acquired active immunity: The individual is exposed to the microbe through contact in the environment and develops antibodies; –Naturally acquired passive immunity: This is the natural transfer of antibodies from mother to child either through the placenta or through breast milk.

–Artificially Acquired active immunity: With this type of immunity, the individual is exposed to a weakened version of the microbe through a vaccine and develops antibodies. –Artificially acquired passive immunity: The individual acquires antibodies from an alternate source that is already immune to the disease, to fight infection until that individual’s body can make its own antibodies.

Antigens and Antibodies An antibody is a specialized protein that recognizes and interacts with a foreign substance called an antigen. –Antibodies are made in B cells, specialized white blood cells. –All antibodies are members of a group of proteins called immunoglobulins (Igs). An antigen is a protein or sugar chain that provokes a highly specific immune response because it is not recognized by the body.

5 classes of antibodies IgG: This antibody is capable of crossing the placenta to give the fetus immunity during gestation. The body produces a larger concentration of these antibodies upon the second exposure to an infectious agent. IgM: The first antibodies made at the beginning of an infection. IgA: This class of antibodies provides localized protection on mucousal membranes. This includes the respiratory tract, GI tract and is also the antibody passed through breast milk.

IgD: The function of this antibody is not well understood at this time. IgE: This antibody is responsible for histamine release during allergic reactions. Scientists are also finding that in countries where people are frequently exposed to intestinal parasites, that IgE seems to provide some protection against parasites. That function is not seen in most residents of the United States.

General Structure of an antibody Heavy Chain Light Chain Antigen Binding Site Variable Region Constant Region Hinge

Antibodies are composed of light and heavy chains, or proteins. The light and heavy chains are bound together at the hinge. Both the light and the heavy chains are divided into a constant region and a variable region. –The constant region means that that sequence of amino acids and protein structure remains the same for all antibodies within the same class. In other words, all IgG antibodies have the same constant region and all IgE antibodies have the same constant regions, but IgG antibodies do not have the same constant region as IgE antibodies.

The variable region is different within the antibody class. So let’s say that a person has had both the chicken pox and the rubella vaccine. The IgG antibodies that their body made for each of those diseases has the same constant region, but the variable region is different. It is specific for the disease it protects against. –The Antibody Binding Site is also specific for the microorganism that causes the disease. It recognizes and binds to the antigens made by the microorganism.

2 Types of Immunity 1.Humoral Immunity: antibody-mediated immunity. –White Blood Cells called “B cells” are responsible for the production of antibodies. –Antibodies primarily defends against bacteria, bacterial toxins, and viruses that circulate freely in body fluids.

2. Cell-mediated immunity: specialized white blood cells called T cells (T lymphocytes). –Regulate activation and proliferation of other immune system cells such as macrophages. –T cells are generally used to attack host cells containing viruses. Now before we continue, you need to understand that T cells and B cells come with specific anitgen binding proteins already on their membranes. Each cell is specific for a different antigen. This differentiation occurs before an individual is even born. That means that when a person is infected with a microorganism the activation of immune cells can’t occur until the cell with the right antigen binding protein comes into contact with the antigen. See Fig

T Cells and Cell-Mediated Immunity Mature T cells reside in lymphoid organs, such as the lymph nodes and spleen. Antigen Presenting Cells (APC), phagocytic cells, encounter an antigen in the body. The APC phagocytoses the foreign organism. Then it breaks it down, or processes the antigen. Essentially it disconnects part of the antigen from the rest of the cell and places it on the outside of its membrane. In this way it “presents” the antigen to other immune cells. It helps the other immune cells to see and recognize the antigen.

Helper T cell the binds to APC at the point where the antigen is presented on the membrane. The binding of the APC and the Helper T cell results in signal sent from the Helper T cell to Cytoxic T cells, macrophages, and B cells. See Figure 15.9 for a great diagram of this process.

Once the Cytotoxic T cells get the message from the Helper T cell, the Cytotoxic T cells destroy target cells on contact. –Cytotoxic T cells destroy target cells by releasing perforin, an enzyme that forms pores (or punches holes) in the target cell membrane, causing it to lyse. Suppressor T cells are generally thought to turn off the immune response when it is no longer needed. In Fig You’ll see two different kinds of APCs, MHC I and MHC II, that activate Helper T cells. MHC I and II are the proteins that mark that cell as belonging to the body. Essentially they mean, “don’t kill me, I belong here!”

In addition to the activation of the cytotoxic T cells macrophages are also activated to help clean up the infection. B cells are activated to make antibodies by the T Helper cells. Often B cells can’t begin antibody production without the help of the T Helper cells, Finally, memory T Cells are formed. They are the cells that remember a particular antigen when it is found in the body again and quickly begin making immune cells to prevent re-infection.

B Cells and Humoral Mediated Immunity A specific B cell binds to the invading microorganism. It then processes it and presents the antigen in a way similar to the APC. For most B cells to become active they need help from the Helper T cells. The activated Helper T cell (previous activated through the APC) binds to the antigen presented on the B cell and activates the cell for antibody production.

Once the B cell has been activated it will make two different types of cells: –1. Plasma cells: These cells actively make antibody and release it to fight invading microorganisms. Antibodies help to fight infection in several ways. One way is by completely covering the invading microorganism with antibodies. Your book uses the analogy of providing a handle on a slippery object to help the phagocytes get a better grip on it. They can also work to bind many microorganisms together at one time until they can be phagocytosed. Antibodies also bind to the outside of viruses, covering all the attachment proteins on their outer surface preventing them from binding to host cells.

2. Memory B cells: These cells are made so that upon exposure to the same antigen in the future, antibodies can be quickly made before disease can occur. See Fig for a review of Ab mediated and cell mediated immune system and how they work together to protect the body. I find it amazing how much the body does to prevent infection on a daily basis. What is even more amazing is that microorganisms find ways to circumvent this amazing defense! I guess that leads us back to the constant struggle of “survival of the fittest”.

Next Monday we will cover disorders of the immune system and vaccination. No hw this time.