Immune Systems

Gas exchange occurs in the alveoli. Incoming air is taken in by the contraction of the diaphragm with air taken in. The oxygen diffuses across the thin alveolar membrane, which has fused with the capillary membrane so that the gases can exchange across the combined “respiratory membrane”. The alveoli contain surfactant that maintains the opening of the alveoli so that they do not collapse. The alveoli provide a very large surface area for gas exchange in mammals.

IMMUNE SYSTEMS

There are two main immune systems in mammals with the innate immune system being older evolutionarily-speaking. Higher animals will have both an innate immune system and an adaptive immune system. Together, their role is to defend against pathogens. Pathogens for higher organisms include microorganisms such as bacteria, protists, and fungi. These can be found in the water, in the air, and on surfaces.

The goal of the immune system is to recognize self from non-self and to get rid of those things that are recognized as non-self. The innate immune system functions since birth and is relatively nonspecific when it comes to its activity against pathogens. The adaptive immune system stores information about past infections, mounting pathogen-

specific defenses that are remembered for long periods of time—usually the lifetime of the individual.

The innate immune system involves both chemical and physical barriers that act as the first line of defense against pathogens. It does not respond to vaccinations as it has no memory. It starts with having barriers, such as the skin, respiratory tract, mucous membranes, and the GI tract. These types of systems have been in place evolutionarily for about a billion years. Things like tears and mucus will trap pathogens and will remove them from the animal’s system. If a microorganism breaches the skin or other barriers, there are other defenses. The stomach provides a chemical barrier by having a low pH that kills many pathogens. The blood-brain barrier will prevent the pathogens from entering the brain. Urination will help flush organisms out of the bladder.

When a pathogen truly enters the body, the animal will have the ability to detect pathogen-associated molecular patterns or PAMPs, which are the different receptors and proteins on pathogens that characterize them as being foreign to the self. These PAMPs are found on viruses, bacteria, and parasites. Macrophages and related cells have phagocytic properties; that is, they can eat pathogens they recognize as foreign in order to get rid of them as soon as possible. Macrophages are a key player in the innate immune system. They come from monocytes, which is a type of white blood cell that can move into tissues when necessary.

The main thing that the innate immune system does is to activate the inflammatory response in response to a breach in the barrier system or to a pathogen that has entered the tissues. Cells that participate in this include the following:

• Mast cell—these cells dilate blood vessels and release histamines and heparin as inflammatory mediators. They help recruit neutrophils and macrophages to the breached area.

• Macrophage—these are phagocytic cells that kill pathogens and cancerous cells.

• NK cells—these are natural killer cells that kill virally-infected cells and cancerous cells.

• Dendritic cells—these will phagocytize the pathogens and will put out antigens from the pathogen, presenting them to cells of the adaptive immune system.

• Neutrophil—these are roving cells that can exit the bloodstream to go to the site of infection in order to participate in phagocytosis.

So, the NK cells, macrophages, neutrophils, and dendritic cells all participate in phagocytosis in order to get rid of pathogens or to present antigens (that come from the pathogens) on their surfaces in order to activate the adaptive immune system.

Cytokines are also important in the immune system of animals. These are basically chemical messengers that regulate the proliferation of cells, cell differentiation, and gene expression in order to affect the immune response. There are forty different cytokines in humans alone. There are the interleukins that involve white blood cell activity, interferons that are released by infected cells to warn nearby cells, and other messenger molecules that cause systemic responses to infection. Cytokines tend to be initially pro-inflammatory so that the signs of inflammation are present (such as redness, swelling, and localized heat).

Other important molecules to this system are the molecules of the complement system, which are proteins that are activated early on in the infection, setting off a cascade of biochemical reactions that mark cells for phagocytosis. They are complementary to the antibody system by marking a pathogen so that both macrophages and B cells can be activated in order to engulf the pathogen. When complement proteins mark the cells for killing, this process is called opsonization. Some complement proteins will form attack complexes on the microbe in order to have pores open up in the microbial cell membranes.

The adaptive immune system involves a slower immune response than the innate immune response but it has memory and is highly specific to a particular pathogen. When antigens are presented to the cells of the adaptive immune system, they are triggered to make antibodies that ultimately mark a pathogen for killing. It is the B cells that make antibodies. There are plasma cells, which are activated B cells, and memory cells that retain the memory of the pathogen so that, years from the infection, the

memory B cells can be activated in order to prevent an infection from developing a second time.

B cells and the adaptive immune system respond to antigens. So, what is an antigen? This can be anything—a carbohydrate or protein or virus particle—that is recognized as foreign and that can have an antibody made to it. The antibodies bind to a specific antigen that they’ve been made against. Once made, antibodies can facilitate the killing of the pathogen by causing their agglutination or by triggering other cells to recognize the pathogen as foreign so it can be killed. Figure 52 shows the route that B cells go through when activated:

https://www.shutterstock.com/image-vector/activation-bcell-leukocytes-lymphoblastmemory-bleukocyte-748304221?src=mkKSbIA4bKMxh6K30mACkA-1-51