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Understanding the Immune System

August 4, 2015

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Key Cells of the Immune System

Some key cells of the immune system are:

  • Neutrophils
  • Dendritic cells and macrophages
  • Helper and Killer T cells
  • Natural killer cells
  • B cells


Neutrophils are one of the "first-responders" in the body's response to infection. Normally, you have more neutrophils than any other type of white blood cell. They circulate in the bloodstream and are your main defense against bacteria. They eat bacteria and produce toxic chemicals that destroy them. These are the cells that give pus its whitish/yellowish color.

Dendritic Cells and Macrophages

Dendritic cells are found mostly in the skin and mucous membranes that protect the openings of the body (e.g., nose, mouth, and throat). These cells capture and carry invaders to the lymph nodes or spleen. Macrophages (their name comes from Latin and means "big eaters") protect different organs, including the intestines, lungs, liver, and brain. Like dendritic cells, macrophages capture and carry invaders to the lymph organs.

These two types of white blood cells are known as scavengers. They engulf (eat) foreign invaders, break them apart, and display pieces of the germs -- known as antigens (from antibody-generating) -- on their surfaces. The body can then make antibodies to that specific germ, which helps the body get rid of that invader faster and remember it in the future. These cells also produce chemical messengers (known as cytokines) that instruct other immune cells to go into action.

T Cells

Once antigens are processed and displayed on the surface of macrophages, they can be recognized by helper T cells (also known as CD4 cells). When CD4 cells "see" the antigens displayed, they get busy and put the word out to other immune system cells. In other words, these cells coordinate and direct the activity of other types of immune cells -- such as killer T cells, B cells, and macrophages -- calling them into action to fight the intruder. CD4 cells produce many different cytokines in order to communicate effectively with other immune system cells.

Killer T cells directly attack and destroy cells infected by viruses as well as abnormal cells that may become cancerous. Suppressor T cells call off the immune system attack once the invader is conquered. This is to make sure the killer T cells stop killing once their job is done. Both killer T cells and suppressor T cells are also known as CD8 cells.

Natural Killer Cells

Natural killer, or NK cells also destroy infected or abnormal cells. NK cells are an important part of the early response to viral infections because they kill cells infected by virus while the body is recruiting killer T cells into action.

B Cells and Antibodies

B cells are another type of immune cell that is activated by CD4 cells. When a B cell recognizes an antigen, it goes into action and produces antibodies (also called immunoglobulins). An antibody is a protein that attaches to an antigen like a key fits a lock. Each antibody matches a specific antigen. When an antibody has matched up with an antigen, it has marked the intruders for destruction by scavenger immune cells. Antibodies also activate a complex chemical reaction that destroys bacteria by punching holes in their outside surface (the bacterial membrane), which kills them.

When you are exposed to a germ for the first time, it usually takes a while (several weeks to a few months) for your body to produce antibodies to fight it. But if you were exposed to a germ in the past, you will usually still have some B cells (called memory cells) in your body that recognize or 'remember' the repeat invader. This allows the immune system to go into action right away. This is why people get some diseases, such as chickenpox or measles, only once. They may get chickenpox or measles germs in their bodies more than once, but their bodies remember the germs and react so quickly that they do not get sick.

This is also how vaccines work -- they introduce your body to an inactive form of a particular germ and trigger your immune system to produce antibodies to that germ. The inactive form prevents you from getting sick while still allowing your body to make important memory B cells. This is why the expression "vaccinated against" some disease (e.g., smallpox) is used.

HIV and the Immune System

The antibodies your body creates in response to HIV do not protect you against HIV like antibodies for other infections. Instead, HIV turns the CD4 T cells that would kill it into factories for making more copies of itself. This causes CD4 cells to lose the ability to communicate with the rest of the immune system. Without CD4 cells organizing the rest of the immune system, important immune cells do not know which invaders need to be removed from the body. When the immune response is not properly activated and organized, people are at risk for opportunistic infections (OIs) and cancers that usually do not harm people with healthy immune systems. This is why, in the case of HIV, being antibody positive does not mean you are protected, but rather that HIV is weakening your immune system and using it to make copies of itself.

HIV can also infect macrophages and other immune cells. Your immune system recognizes and produces antibodies to HIV, but antibodies alone are not enough to eliminate the virus. This is partly because HIV changes or mutates faster than the immune system can respond to it.

HIV persists in the body by forming a "reservoir." The HIV reservoir refers to a collection of inactive, 'resting,' or latent HIV-infected cells. Researchers have reported evidence suggesting that HIV-infected cells can persist and expand by genetically cloning or copying themselves. In other words, elimination of HIV in the body (complete cure) will require not only that HIV be eliminated from the bloodstream, but also that we find a way to prevent these latent cells from multiplying or empty the reservoirs altogether.

There are several known reservoirs, including immune cells in the gut, lymphoid tissue, blood, the brain, the genital tract, and bone marrow. It is unclear when reservoirs are established, but recent research suggests that it could be as early as 24 hours after initial infection.

Other problems may result from limiting the activity of the bone marrow, which can occur as a side effect of certain HIV drugs like Retrovir (zidovudine). Since the bone marrow is where immune cells are produced, if its activity is reduced you may have lower numbers of immune cells available, which again may cause you to be more vulnerable to infections.

The good news is that early treatment with a combination of HIV drugs can minimize the size of the reservoir. In addition, newer HIV drugs have fewer side effects and are more effective at stopping the virus from multiplying (making copies of itself) and infecting more CD4 cells. Since CD4 cells are key to a healthy immune response, this can give your immune system a fighting chance to replenish its supply of CD4 cells and to defend itself (you!) against opportunistic infections.

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This article was provided by The Well Project. Visit The Well Project's Web site to learn more about their resources and initiatives for women living with HIV. The Well Project shares its content with to ensure all people have access to the highest quality treatment information available. The Well Project receives no advertising revenue from or the advertisers on this site. No advertiser on this site has any editorial input into The Well Project's content.


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