 We are going to talk about immunology. Immunology deals with host defense systems. Most organisms are under constant attack from pathogens. There are pathogens all around us looking for an opportunity to invade and enjoy the bounties our bodies have to offer. Over the course of evolution, both the warring parties have developed several strategies for the conquest and defense. Since survival is at stake, new offensive and defensive weapons are being constantly developed by both to overcome the other. There are several features of our bodies that fall in the category of defensive systems that we generally do not give due credit. These strategies that we have evolved are generally nonspecific. Nonspecific strategies are referred to as innate immune system. They can be very simple. For example, our skin is a defensive barrier that hinders the pathogen invasion. You may have noticed that when skin is lost due to an injury, infection is one of the primary concerns. It generally follows an injury. That's why we apply antibiotic ointments to the wounds to prevent bacteria from growing there and causing an infection. Gastric juice in our stomach, which is there to generally to degrade the food that we eat, also can destroy pathogens that may cause infection of the digestive system. Our tears and saliva have enzymes that can destroy many organisms. You may have noticed that when cats or dogs get a wound, they lick their wounds. The reason is that saliva contains an enzyme called lysozyme that enzyme can degrade the cell wall of many bacteria. When these animals are actually licking their wounds, they are doing the same thing what we do when we apply antibiotic to our wounds. They are applying antibiotic to their wounds when they lick their wounds. Sineasing also can remove the potential pathogens from our respiratory tract. Normal flora, as we know that there are many bacteria that grow in our intestinal tract in our digestive system and that they are there normally and they help us digest several things. They can also restrict the growth of harmful pathogens. The best way to understand immune response is by drawing an analogy with a war, enemy forces advancing on the territory of the opponent. I don't know how many of you have lived through a war. I have witnessed a couple and I can tell you with certitude that the first response to an attack is the blowing of the sirens to alert the forces that attack has been detected and also alerts the population, the civilian population to take cover since an attack is coming. The siren is the first response whether the attack is from air, from sea or of course from land. After an attack has been detected and announced, the first line of defense is moved forward. The first line of defense is generally highly mobile, lightweight, compact units that have one goal and that one goal is to stall the enemy, allowing giving time to heavy armament to move in for the defensive purposes and also giving time to the warring council to come up with a tactical strategy to neutralize the hostile forces. So what are the decisions our immune systems have to take when an attack has been detected? So there are four features of our immune system that are very important in neutralizing the invading forces. Number one is specificity. So for example, if an infection by Salmonella has been detected, our immune system should generate forces that can combat Salmonella and Salmonella only. One of the things that our immune system generates, I'm sure you're aware of it, are antibodies and it takes energy to generate antibodies. So if Salmonella has been detected in our bodies, we will make antibodies against Salmonella only. It will be wasteful to make antibodies against polio virus, E. coli, other bacteria. So our immune system has specificity. It has diversity. That's the second thing. It doesn't matter what was the source of a pathogen. Where did it come from? Our body has to have all type of armaments capability to develop them, should have them at hand. No matter what pathogen enters, it should be able to produce a response against it. So our bodies can produce millions of different types of antibodies against potential pathogens. Our immune system has also a memory. That's the third feature. So once a pathogen has been detected, our immune system remembers that our body was attacked by this pathogen. They have reserve forces specifically to combat that particular pathogen. So that is a beauty in our immune system that it can remember and it keeps forces ready for that particular pathogen. Additionally, the fourth very important feature is our immune system is able to recognize self from non-self. It will be catastrophic because our immune system, make no mistake about it, is geared to destruction. So if that destructive force is focused on our body, it will be very catastrophic. So our immune system has to have ability to recognize our body from the other entities, from foreign entities that can cause potential harm. We will continue our discussion of immune system in the next module.