The Immune System: Your Body's Amazing Superheroes

Hey Super Kids! Did you know you have an incredible team of superheroes living right inside your body? They're called your immune system, and their job is to protect you from all the tiny bad guys that try to make you sick!

Who are the Bad Guys?

Imagine tiny villains like viruses, bacteria, fungi, and other parasites. These are called pathogens, and they're always trying to get into your body. But don't worry, your superheroes are on patrol!

Your First Line of Defense: Innate Immunity (Your Body's General Guards!)

Even before your main superheroes show up, you have some awesome general guards protecting you. This is called innate, or natural, immunity, and it's like a shield that every animal has.

• Body Fluid Fighters: Your body has special liquids, like your blood, with substances that can stop bad guys from growing or even clump them together so your body can get rid of them!

• Pac-Man Cells! You also have cells called phagocytes that literally "eat" up the bad guys by ingesting and degrading them. And there are "natural killer cells" that can destroy certain cancerous cells.

These general guards are nonspecific, meaning they fight any bad guy that tries to get in, not specific invaders. They're always ready for action!

Your Super Smart Heroes: Adaptive Immunity (The Elite Squad!)

Only animals with backbones (like us humans!) have an even smarter team of defense mechanisms called adaptive immunity. These heroes are super special because they can recognize and destroy specific substances! When your adaptive immune system jumps into action, its defensive reaction is called the immune response. And any substance capable of generating such a response is called an antigen, or immunogen. Antigens are substances in microorganisms or tissues, like toxins or enzymes, that the immune system considers foreign.

Here's what makes this team so cool:

1. Super Specific: Immune responses are normally directed against the antigen that provoked them and are said to be antigen-specific. This specificity is one of the two properties that distinguish adaptive immunity from innate immunity.

2. Amazing Memory: This is the best part! The other distinguishing property is called immunologic memory. This is the ability of the adaptive immune system to mount a stronger and more effective immune response against an antigen after its first encounter with that antigen, leaving the organism better able to resist it in the future. That's why you usually only get chickenpox once!

Meet the Lymphocytes: The Heart of the Team!

The main superheroes of your adaptive immune system are special white blood cells called lymphocytes.

• How they know the bad guys: A lymphocyte is different from all other cells in the body because it has about 100,000 identical receptors on its cellular membrane that enable it to recognize one specific antigen. The receptors are proteins containing grooves that fit into patterns formed by the atoms of the antigen molecule—somewhat like a key fitting into a lock—so that the lymphocyte can bind to the antigen. When an antigen invades the body, normally only those lymphocytes with receptors that fit the contours of that particular antigen take part in the immune response.

• Making more heroes: When they do, "daughter cells" are generated that have receptors identical to those found on the original lymphocytes. The result is a family of lymphocytes, called a

  lymphocyte clone, with identical antigen-specific receptors. A clone continues to grow after lymphocytes first encounter an antigen so that, if the same type of antigen invades the body a second time, there will be many more lymphocytes specific for that antigen ready to meet the invader. This is a crucial component of immunologic memory.

Two Kinds of Lymphocyte Heroes: B Cells and T Cells!

Your body makes lymphocytes from special stem cells in your bone marrow. Then they get their training!

• B Cells (The Antibody Factories!): Some lymphocytes are processed in the bone marrow and then migrate to other areas of the body—specifically the lymphoid organs. These lymphocytes are called

  B lymphocytes, or B cells (for bone-marrow-derived cells). When a B cell is stimulated by an antigen that it encounters in the body fluids, it transforms, with the aid of a type of T cell called a helper T cell, into a larger cell called a blast cell. The blast cell begins to divide rapidly, forming a clone of identical cells. Some of these transform further into

  plasma cells—in essence, antibody-producing factories. These plasma cells produce a single type of antigen-specific antibody at a rate of about 2,000 antibodies per second.

  • What antibodies do: These products are called antibodies and belong to a special group of blood proteins called immunoglobulins. The antibodies then circulate through the body fluids, attacking the triggering antigen. Antibodies attack antigens by binding to them. Some antibodies attach themselves to invading microorganisms and render them immobile or prevent them from penetrating body cells. In other cases, the antibodies act together with a group of blood proteins, collectively called the

    complement system. In such cases, antibodies coat the antigen and make it subject to a chemical chain reaction with the complement proteins. The complement reaction either can cause the invader to burst or can attract scavenger cells that “eat” the invader.

  • Memory B cells: Not all of the cells from the clone formed from the original B cell transform into antibody-producing plasma cells; some serve as so-called memory cells. These closely resemble the original B cell, but they can respond more quickly to a second invasion by the same antigen than can the original cell.

• T Cells (The Direct Fighters and Helpers!): Other lymphocytes move from the bone marrow and are processed in the thymus. These lymphocytes are called

  T lymphocytes, or T cells (for thymus-derived cells). There are two major classes of T cells produced in the thymus: helper T cells and cytotoxic, or killer, T cells.

  • Helper T Cells: Helper T cells secrete molecules called interleukins (abbreviated IL) that promote the growth of both B and T cells.

  • Cytotoxic (Killer) T Cells: Cytotoxic T cells destroy cells infected with viruses and other pathogens and may also destroy cancerous cells. Cytotoxic T cells are also called suppressor lymphocytes because they regulate immune responses by suppressing the function of helper cells so that the immune system is active only when necessary.

  • T Cells' Special Sight: The receptors of T cells are different from those of B cells because they are “trained” to recognize fragments of antigens that have been combined with a set of molecules found on the surfaces of all the body’s cells. These molecules are called MHC molecules (for major histocompatibility complex). As T cells circulate through the body, they scan the surfaces of body cells for the presence of foreign antigens that have been picked up by the MHC molecules. This function is sometimes called immune surveillance.

The Big Fight: How Your Immune System Responds!

When an antigen enters the body, it may be partly neutralized by components of the innate immune system. It may be attacked by phagocytes or by preformed antibodies that act together with the complement system. Often, however, the lymphocytes of the adaptive immune system are brought into play.

The human immune system contains approximately 1 trillion T cells and 1 trillion B cells, located in the lymphoid organs and in the blood, plus approximately 10 billion antigen-presenting cells located in the lymphoid organs. To maximize the chances of encountering antigens wherever they may invade the body, lymphocytes continually circulate between the blood and certain lymphoid tissues.

If lymphocytes encounter an antigen trapped by the antigen-presenting cells of the lymphoid organs, lymphocytes with receptors specific to that antigen stop their migration and settle to mount an immune response locally. As these lymphocytes accumulate in the affected lymphoid tissue, the tissue often becomes enlarged—for example, the lymph nodes in the groin become enlarged if there is an infection in the thigh area.

Antigen-presenting cells degrade antigens and often eliminate them without the help of lymphocytes. If there are too many antigens for them to handle alone, however, the antigen-presenting cells secrete IL-1 and display fragments of the antigens (combined with MHC molecules) to alert the helper T cells. The IL-1 facilitates the responsiveness of T and B cells to antigens and, if released in large amounts (as it is in the course of infections), can also cause fever and drowsiness.

Helper T cells that encounter IL-1 and fragments of antigens transform into cells called lymphoblasts, which then secrete a variety of interleukins that are essential to the success of the immune response. The IL-2 produced by helper T cells promotes the growth of cytotoxic T cells, which may be necessary to destroy tumorous cells or cells infected with viruses. The IL-3 increases the production of blood cells in the bone marrow and thus helps to maintain an adequate supply of the lymphocytes and lymphocyte products necessary to fight infections. Helper T cells also secrete interleukins that act on B cells, stimulating them to divide and to transform into antibody-secreting plasma cells. The antibodies then perform their part of the immune function.

The process of inducing an immune response is called immunization. It may be either natural—through infection by a pathogen—or artificial—through the use of serums or vaccines. The heightened resistance acquired when the body responds to infection is called active immunity.

Passive immunity results when the antibodies from an actively immunized individual are transferred to a second, nonimmune subject. Active immunization, whether natural or artificial, is longer-lasting than is passive immunization because it takes advantage of immunologic memory.

So, next time you feel a little under the weather, remember your incredible immune system is on the job, fighting hard to keep you healthy and strong! They're your very own personal superhero team!