Understanding the Immune System, Part I

How do vaccines interact with the immune system?

The immune system and protection from disease

The immune system is the set of defenses in the body that protects us from becoming ill. It is made up of many different types of cells and substances, all of which work together to help us heal when we have been injured, get well when we have become ill, and avoid some illnesses altogether.

The immune system can do this because it is able to recognize, fight and remember foreign invaders, like bacteria or viruses, which can cause illness when they enter the body. Such invaders are called "pathogens." A common cold is caused by a pathogen (a cold virus). HIV is the pathogen that causes AIDS.

When a new pathogen enters the body, the immune system uses a variety of defenses to control or get rid of it. One of the first responses comes from B cells. These cells can recognize foreign invaders soon after they have entered the body, but before they have entered and infected any of the body’s cells. Many pathogens, including HIV, enter cells and infect them in order to multiply.

B cells produce antibodies which coat the surface of the pathogen to stop it from multiplying itself or infecting cells. This process is called "neutralization." Antibodies also label the pathogen so that other immune defenses can "see" and attack it.

Another initial response comes from other immune system cells called dendritic cells and macrophages. These cells patrol the body and pick up the pathogen. They then carry the pathogen to the lymph nodes, which are the hubs of the immune system. Lymph nodes can be found under the jaw, under the arms, in the gut and in the groin. When we start becoming ill, our lymph nodes often become swollen or sore as immune cells gather in the nodes to fight the infection.

In the lymph node, the patrolling cells show or "present" the pathogen to CD4+ T cells. These "helper" CD4+ T cells coordinate the activities of a set of "killer" cells called CD8+ T cells. CD4+ and CD8+ T cells work together to eliminate cells that have been infected by pathogens.

HIV infects and kills CD4+ T cells, which is why doctors sometimes count these cells when people are infected with HIV. Our immune systems try to fight off HIV by sending CD8+ T cells to kill off the HIV-infected CD4+ T cells. Unfortunately the immune system cannot eliminate HIV from the body. Over a period of time, HIV infection exhausts the body’s immune defenses. This leaves HIV-infected people vulnerable to a variety of other infections. Antiretroviral drug treatment can suppress multiplication of the virus in the body and so reduce HIV-related illness, prolonging the life of the infected person. But this treatment cannot rid the body of HIV completely.

Immune memory

Although the immune system cannot control HIV, it can control or get rid of many other infections. This is why we become well after many illnesses. After a pathogen has been controlled, most of the immune cells and antibody that fought the infection disappear. However a small group of "memory" immune cells remains in the body. These memory cells have already fought the pathogen once before and so if the pathogen ever enters the body again they can very quickly start a strong immune response. Memory cells "arm" the body against future infections from the same pathogen. There are some infections, such as chickenpox or measles, which we generally get only once. This is because memory cells from the first infection effectively fight the pathogen if we are ever exposed to it again.

Vaccines and immune memory

Immune memory is a key reason why vaccines protect us from disease. An effective vaccine safely introduces the immune system to a pathogen that it has never seen before. It arms the immune system so that it can effectively control the pathogen if it ever invades the body. Vaccines use safe forms or fragments of pathogens to mimic the actual pathogen and trick the body into generating immune responses. The fragments or safe forms of pathogens that are used in vaccines are called "immunogens." This word reflects the fact that vaccines cause immune responses, not disease.

When the vaccine enters the body, the immune system sees it and responds to it just as it does to any foreign substance. T cells and B cells react to the vaccine. Some of these cells become memory cells. These cells are ready to respond to the real pathogen if it ever enters the body.

All of the AIDS vaccines in development today use small fragments of HIV as their immunogens. These fragments cannot cause HIV infection. The goal of these experimental AIDS vaccines is to produce memory cells that will be able to mount a rapid, strong immune response against HIV if a person is ever exposed to whole, live HIV through high-risk contact such as unprotected sex.

Today the challenge for AIDS vaccine developers is to identify the best immunogens to create strong antibody and cellular responses that will protect against HIV infection and disease.