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Your Immune System is your Body’s Defense Against Infection

You may already know that vaccines work by stimulating the body’s own immune system to protect you against illness or infection. The first step to understanding how that works is to understand how your body functions naturally, from the development of infection to the resulting immunity.

An infection is caused when germs like bacteria or viruses enter the body, and attack and multiply. The immune system has strategies for fighting infection, which involves the different blood cells. Blood contains two types of cells – red blood cells, which deliver oxygen to tissues and organs, and white or immune cells, which serve to fight and protect against infection. The infection-fighting white cells consist primarily of macrophages, B-lymphocytes, and T-lymphocytes.

  • Macrophages are white blood cells that digest germs, and dead or dying cells. During the germ-digestion process, macrophages leave behind parts of the invading germs called antigens. The body identifies antigens as dangerous and stimulates antibodies to attack them.
  • B-lymphocytes are white blood cells that produce antibodies to defend against the antigens left behind by the macrophages.
  • T-lymphocytes are white blood cells that attack cells in the body that have already become infected. When the body is exposed to a germ for the first time, it can take several days for the germ-fighting process to defend against the infection. In the meantime, cells will become infected and the T-lymphocytes kick in and attack the infected cells.

After the infection clears, the body keeps a few T-lymphocytes, called memory cells, which take defensive action quickly if the body encounters the same germ again. When the immune system detects familiar antigens, B-lymphocytes produce antibodies to attack them.

Vaccines initiate this process and help develop immunity by simulating an infection and causing the immune system to produce T-lymphocytes and antibodies. Normally, the infection simulated by a vaccine does not cause illness; however, sometimes the imitation infection results in minor symptoms, like fever. These minor reactions are a normal by-product of the body’s immunity- building process. Vaccines can also sometimes produce injection-site reactions.

It typically takes a few weeks after vaccination for the body to produce T-lymphocytes and B-lymphocytes. This is why it’s important to be vaccinated in advance of any potential exposure – it’s possible that a person vaccinated just before or just after exposure to a live infection could become ill, regardless of the vaccination.

Natural Immunity

Some people believe that natural immunity acquired from actually getting a disease is “better” than the immunity provided by vaccines. While there are some advantages to the duration of naturally acquired immunity, it is important to note that natural infections can cause severe complications, and can be even deadly in some cases. There is no way to predict who will develop serious complications from “mild” diseases, such as chickenpox.

Many vaccine-preventable diseases are rare in North America, but we do see resurgence when vaccine rates drop, such as with recent measles outbreaks in the U.S.A. and Canada. Also, some diseases that are rare in North America are far more common elsewhere in the world, and can be brought home, putting unvaccinated people (especially children, the elderly, and the immune-compromised) at risk. Even with advances in health care, the diseases that vaccines prevent can still be very serious – and vaccination is a good way to prevent them.

Future Vaccines

In most of the developed countries of the world, there is a comprehensive schedule of available vaccines, started in infancy, which protect people from most of the major childhood diseases. There are also many adult vaccines available for travelers, and for protection against flu, shingles, cervical cancer, tetanus, and a host of other diseases.

Additionally, vaccines are being developed for a number of serious diseases that we see around the globe. The World Health Organization (WHO) vaccine pipeline tracker lists the following vaccines in development:


Month of most recent update

Updating Partner


May 2018



May 2018

Schwartz Consulting


May 2018



May 2018

WHO Secretariat


May 2018



May 2018


Shigella and ETEC

May 2018



January 2019

WHO Secretariat

Lassa, MERS-CoV, Nipah

May 2018



May 2018


Pneumococcal Infections

May 2018

MSF Epicentre


In next month’s blog, we discuss the different types of vaccines, and how the type may affect the need for additional doses.


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