The immune system is the body’s defense system that protects us from foreign (non-self) substances. The immune system assumes foreign things are dangerous as they are usually pathogens like bacteria, viruses, fungi, and parasites. The immune system also has defensive functions against other disease processes like cancer.

When the body has an immune system that is functioning at just the right level we remain healthy. However, if the immune system is hypoactive or hyperactive disease can form. When the immune system is not active enough we lose our defense against pathogens and cancer. When the immune system is too active, the immune system itself can cause disease by damaging the body.

There are two main types of immunity, Innate and adaptive. You can think of the innate immune response like a shotgun. It isn’t very specific to the target, but it can be fired quickly without wasting time trying to aim perfectly. In contrast, the adaptive immune response is like a meticulous sniper. He/she needs a lot of time to plan his attack and set up his rifle, but then their attack is very specific to the target and more effective.

Innate immunity is a fast acting, generic immune response carried out mainly by the complement system and acute inflammation (neutrophils and macrophages). It recognizes certain characteristics that are conserved across a variety of related microbes. In this way, the Innate Immune System can recognize a pathogen is foreign and potentially dangerous without having to figure out exactly what kind of pathogen it is.  This also allows the system to identify a bunch of different pathogens by looking for a very small number of common characteristics. These recognized characteristics, called Pathogen Associated Molecular Patterns (PAMPs), are often lipids or sugars on the surface of pathogens. When a PAMP is identified by the Innate Immune System a generic immune response is mounted that would work against a variety of pathogens with that same PAMP. The non-specific recognition and response of the Innate Immune System allows for a reaction that is immediate. No time is wasted trying to figure out exactly what type of pathogen it is before an attack is mounted. Physical barriers in the body like skin are also technically part of the Innate Immune System. These barriers usually aren’t the focus when discussing this topic, but they are very important because they keep a huge majority of pathogens we come into contact with from ever getting into the body.

The Innate Immune System is often adequate to prevent infection and remove a pathogen on its own. However, when innate immunity can’t remove a pathogen on its own the Adaptive Immune System is activated to finish the job. Adaptive Immunity is mainly comprised of lymphocytes (B & T cells). It is a more specific and potent response, but has a slower onset because this targeted attack takes time to create.

Immunologic memory is when the immune system has a quicker response to later exposures of the same pathogen. It is only present in Adaptive Immunity. A small number of the lymphocytes involved in the initial adaptive response can remain in the body for decades after the pathogen has been removed. The next time the same foreign substance is recognized, these remaining lymphocytes get activated quickly because they have already seen the pathogen. The immunologic memory of the adaptive immune system is the function by which vaccines work. You are exposed to a foreign particle on purpose so that if that particle is encountered later your body already knows how to fight it.

There are two main ways to destroy a pathogen, Humoral Immunity and Cell-Mediated Immunity. The Adaptive and Innate Immune Systems each use humoral and cell mediated. In Cell-Mediated Immunity cells, such as CD8 T-cells and neutrophils, are directly involved in attacking the pathogen. This usually involves phagocytosis (immune cell engulfing pathogen) or cytotoxicity (immune cell triggers cell with intracellular pathogen to undergo apoptosis). Technically, the release of cytokines to recruit other immune cells is also considered cell-mediated, but it is usually not the focus in this context.

Humoral Immunity involves acellular macromolecules which are dispersed in the body’s fluids. So living cells do not act directly on the pathogen. The immune cells indirectly attack the pathogen by making acellular molecules which directly affect the pathogen. There are many molecules like complement that fall under this category, but usually when Humoral Immunity is mentioned it is referring to antibodies. Antibodies are molecules released by B cells that act directly on the target pathogen. Humoral Immunity primarily targets extracellular pathogens like bacteria.

Pictures Used:

• “Shotgun Rifle Gun Weapon” available at http://pixabay.com/en/shotgun-rifle-gun-weapon-146722/ via public domain
• “H&K PSG-1 Sniper Rifle” by Jeswalo Chen available at http://commons.wikimedia.org/wiki/File:H%26K_PSG-1_Sniper_Rifle.jpg via Creative Commons Attribution-Share Alike