Immune System Overview

Topics of this page: Important Terms, General Response

Important Terms You Should Know About the Immune System:

The immune system is composed of white blood cells, and there are three basic classes of white blood cells that you should know in order to understand the immune system.

The first are phagocytes. These are cells that destroy other cells by surrounding them and engulfing them; a process called phagocytosis. Macrophages (or antigen presenting cells) are a type of phagocyte that travel through the body and engulf anything that is not recognized as part of the self. Inside the macrophage, an organelle called the lysosome, breaks down the engulfed substance into its component parts. These antigens are then presented on the surface of the macrophage bound to membrane proteins called Class II major histocompatibility complex antigens (MHC Class II). An antigen is anything that your body recognizes as foreign.

The last two important classes are both lymphocytes. T Cells (or T lymphocytes) are made from stem cells in the bone marrow, and then travel to the thymus, a small gland that sits above the heart. There they learn to recognize invading microbes by the antigens that are exposed on their surface. There are four main types of T cells. Helper T Cells are responsible for initiating the immune response. They contain a receptor, called a T-cell receptor (TCR), which can bind to a MHC Class II that is presenting a peptide in its binding site. Cytotoxic T Cells are responsible for rupturing membranes of cells that have been infected. Inducer T Cells oversee the development of T cells in the thymus and Suppressor T Cells are responsible for stopping the immune response.

The second kind of lymphocytes are the B Cells. These cells do not travel to the thymus after they have been made like the T cells do. Instead, they travel in the blood stream "looking" for foreign antigens. When a B cell "sees" an antigen, it divides rapidly into plasma cells which each produce an antibody. The antibody will bind to the foreign invader wherever it occurs in the body and mark it for destruction.

General Response of Immune Cells to Infectious Agents:

Normal Immune Response:

In a normal immune response, when an invading microbe enters the body, it is engulfed by a macrophage. It is then broken down by the lysosome into its constituent parts, or peptides. These peptides are then presented on the surface of the cell by the MHC Class II. The helper T cell can now recognize this foreign structure and binds to the macrophage at the place where the MHC Class II is presenting the antigen, which it learned to recognize during development (the TCR binds to the antigen that it is specialized to see). This causes activation of the helper T cells and the immune response is initiated. B cells, which recognize certain antigens, also bind to another part of the antigen on the macrophage. The T cells then release lymphokines, causing the B cells to multiply and mature into the plasma cells that make antibodies to mark the peptide for destruction.

Autoimmune Response:

An autoimmune response occurs because, for some reason, the helper T cells recognize a cell of the body (or self cell) as foreign, and mark it for destruction. We are normally protected from this occurrence during the development of the immune system. Usually, in utero, all of the T cells that have receptors for self-peptides are killed by the thymus. If this doesn't happen as it should, a T cell with receptors for self-peptides can get out into the blood stream. There it can recognize a self-peptide and initiate an immune response against the body. This is what occurs in Myasthenia Gravis.

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