Immune system basics

THE BASICS
The immune system can be divided into two parts: the innate (aka non-specific) part and the acquired (aka specific) part. In most textbooks, the acquired part of the immune system receives the most coverage, but the innate part carries most of the weight, so we'll start there.

The non-specific part of the immune system consists of all of the things that keep pathogens (germs) out of the body. It works similarly to the walls and moat surrounding a castle, which work to keep the enemy soldiers out. The non-specific part of the system includes the skin, and also things like the stomach acid (which dissolves any pathogens that we swallow), and lysozyme (which is found in tears, and breaks down pathogens), and mucus (which traps pathogens that we inhale). Most pathogens don't get past these parts of the non-specific immune system. There are also things like fever, and other changes which help the body fight infections, but also make us feel sick, and these can be considered part of the non-specific system as well.

The specific part of the immune system is only activated if a pathogen does sneak past the non-specific part of the system. It is similar to friendly soldiers within the castle walls, whose job is to recognize, hunt down, and destroy any enemies that manage to get inside of the castle. It consists of B cells and T cells, each of which targets a specific pathogen. (A T cell that can target the bird flu virus, for instance, can only target the bird flu virus. A different T cell is needed for chicken pox virus.) When the specific immune system is activated, it also activates those parts of the non-specific immune system (like the fever, etc discussed above) that make us feel sick. (The fever, etc, also serves as a call to arms for the specific part of the immune system.)

TEACHING THE SPECIFIC PART OF THE IMMUNE SYSTEM
The non-specific immune system is always primed and ready - your skin is always there, keeping pathogens at bay. The specific part of the immune system, when we're born, is naive - it can't really respond to anything very well. But, it does learn. Once the specific immune system meets a pathogen for the first time, it remembers it and responds to it much quicker the next time. This is why if the chicken pox virus sneaks past the non-specific part of the immune system once, we get chicken pox. If the chicken pox virus sneaks past the non-specific part of the immune system a second time, the specific part of the immune system recognizes it and pounces on it, killing it before it can really make us sick. We get the chicken pox once, but not twice. (For those of you familiar with shingles and how it relates to chicken pox, I will be getting to that - don't start writing angry comments just yet)

Wouldn't it be great, though, if we could teach the specific part of the immune system to recognize chicken pox without having to actually get the chicken pox that first time? It turns out that we can. We show the immune system a model of the chicken pox virus, and the system learns what the chicken pox virus looks like from the model. The non-specific immune system activates (so we may feel a bit sick, since merely activating the immune system can make us feel sick), and the specific immune system learns to recognize the chicken pox virus. Then, the first time the actual chicken pox virus sneaks past the non-specific immune system, the specific part of the immune system recognizes the virus from the model and pounces on it before it can make you sick.

What is this model? It's a vaccine. The vaccine looks like the actual pathogen, activates the specific part of the immune system (and may make us feel sick briefly) and teaches it to recognize that pathogen, and when the real pathogen comes around, the immune system pounces on it.

Next: The flu shot, and why we need it every year -->

Edited 20 April 2010