What Is the Immune System?


Your immune system protects you from some things and tolerates others.

To maintain health, the balance between a destructive response and a tolerant one has to be just right.

To understand the autoimmune attack of beta cells in type 1 diabetes, it helps to understand how the immune system normally functions. In humans, the immune system protects the person from outside invaders (also known as pathogens), such as bacteria or viruses, and abnormal or diseased cells, such as cancer cells. Additionally, the immune response allows some foreign material and normal cells for each individual (or “self”) to be tolerated. The balance between a destructive response and a tolerant response has to be just right; otherwise, people get autoimmune medical problems.

On this page you will learn about:

Lymphocytes & Immune Organs

The immune cells are called lymphocytes, a type of white blood cells. Important immune organs or sites in the body are the thymus, bone marrow and lymph nodes.

Lymphocytes include:

  • T cells that can attack or kill infected or defective cells, and also regulate the immune response. The T cell receptors (TCRs) on their cell surface recognize and respond to foreign or abnormal tissue. This process is called cell-mediated immunity. See the glossary below for more about the different types of T cells
  • B cells that make antibodies. B cells are involved in humoral – related to the blood – immunity.
  • NK (natural killer) cells that cause cell death.

Immune organs include:

  • The thymus – a gland in the chest that programs the immune system. An important function is to choose and develop T cells that will protect the body and to eliminate T cells that could attack the body.
  • Bone marrow – the source of precursor, or stem cells that can turn into new blood cells.
  • Lymph nodes – small glands throughout the body that trap foreign or diseased cells.

How the system works

The body has a huge repertoire of different T cells, B cells and NK cells that can respond to foreign or abnormal tissue. The immune cells patrol the body and are flagged down by other cells displaying a snippet or fragment of protein or foreign material. The protein snippet is presented by a major histocompatibility complex (MHC) molecule. What happens next depends upon the type of immune cell, protein fragment, foreign material, MHC molecule and presenting cell. And the immune response is different or unique in each individual.

In general:

  • T cells are selected by the thymus gland during the person’s development, and respond to foreign or abnormal protein snippets presented by MHC class I and class II molecules. They will ignore self proteins. Only those T cells with a T cell receptor specific for the protein snippet will respond. Everyone’s set of T cells and TCRs is different. There are multiple types of T cells, each of which have specific combinations of TCRs, and thus have a distinct function in cell-mediated immunity.
  • B cells will respond to foreign protein snippets presented by MHC class I molecules by forming antibodies. B cells make antibodies against a range of different protein combinations and shapes, but normally do not make antibodies against self proteins. The range of antibody responses is unique to each individual and develops over the person’s lifetime.
  • NK cells respond automatically to foreign or diseased molecules by releasing toxic chemicals that destroy cells. This NK response is universal to all humans, which means everyone normally responds to the same things.
  • The MHC class I and class II molecules play a major role in defining each person’s specific immune response. The structure of the MHC protein determines which protein snippets get presented. Different MHC molecules are associated with predisposition as well as protection from autoimmune conditions.

Glossary -Types of T cells:

  • Cytotoxic T cells (Tc cells) contain the CD8 glycoprotein, and their TCRs interact with MHC class I molecules. Cytotoxic T cells are so named because they can destroy diseased (such as cancerous or virally infected) or foreign (e.g., transplanted) cells.
  • Helper T cells (Th cells) contain the CD4 glycoprotein and recognize the antigens presented by MHC class II molecules. When activated, the helper T cells regulate the immune response by releasing cytokines, small signaling proteins that can promote the function of other T cells. They are instrumental in directing CD8 activity.
  • Regulatory T cells (also known as Treg or suppressor T cells) maintain the balance between tolerance and cell killing, or cytotoxicity. They regulate the duration of an immune reaction and suppress autoimmunity. (Your immune system attacking your own body tissue.)
  • Natural killer T cells (NKT cells), in contrast to the other T cells, respond to other types of foreign material that contain carbohydrate and fat (glycolipid) elements. NKT cells release cytokines as well as chemicals that directly destroy cells, and they have characteristics of both helper and cytotoxic T cells.
  • Memory T cells are critical to the immune system’s ability to acquire immunity against foreign or diseased material. Memory T cells can have either CD4 or CD8 markers, and respond to antigens presented by both class I and class II MHC molecules. An infection triggers the creation of antigen-specific memory T cells, which then remain in the bloodstream long after the infection subsides. If the memory T cells are ever re-exposed to their specific antigen, they quickly generate many new effector T cells to combat this familiar pathogen.

©2007-2020 Collective work Martha Nolte Kennedy,
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