Immunity: Antigens

Pathogens & Antigens

The word pathogen is the official word for a disease causing agent. When we consider the immune system we usually are talking about bacteria and viruses, sometimes fugus, and parasites as pathogens. But, the truth is that you are allergic to anyone’s antigensMolecules on the surface of cells that trigger an immune response..  Even you own!  All living cellsThe basic structural and functional units of life. have antigens that are specific to that individual organismA living individual made up of cells.. This includes the cells of bacteria, fungi, and the protein coat of viruses. Actually one Organism usually has many types of antigens not just one. Large moleculesGroups of atoms bonded together. called epitopes can have different places on them where antigens are present. These places are called antigenic determinant. These antigenic determinants are capable of activating the human immune system.

There are also things called haptens. Haptens are disease-causing agents. They need to combine with proteinsLarge molecules made of amino acids with various functions in the body. in your body. This combination is necessary for them to be infectious or allergenic. The most common form of haptens for humans is cosmetics. All cosmetics can bind with proteins in the body. They then become something that causes allergies. Poison Ivy and other poisonous plants secrete haptens causing the characteristics of inflammation that Poison Ivy brings. Some people are allergic to Poison Ivy. They lack the specific protein needed. This protein would bind with the Poison Ivy as a hapten and provoke the immune system. So whether you’re an epitope, an antigenic determinant, an antigen, a pathogen, or a hapten, you provoke an immune response. This response comes from the human immune system.

Antigens

Antigens seem to be just cell surface proteins inserted in the plasma membraneThe outer boundary of a cell that controls what enters and exits. of cells. This is very similar to the A protein, B protein, and Rh protein. These proteins are inserted into the plasmaThe liquid component of blood. membrane of red blood cells. They give a human their characteristic blood type. You, as an individual, have a unique set of self antigens. It is called your major histocompatibility complex or MHC. This is determined by genes on chromosome 6. Your MHC proteins are different than mine. This is because your version of that gene is different than my version of that gene.

All of your cells have your MHC proteins inserted into their plasma membrane. These MHC proteins are constantly displaying proteins that the cell has made. The proteins that the cell makes are kind of like an identification tag. The MHC protein displays that identification tag. This prevents dendritic cellsImmune cells in the epidermis that help fight infections., macrophages, and B cells from consuming your own cells.

If your cells are infected by a pathogen, they will display altered self-proteins on these MHC proteins. The same occurs if the cells are cancerous. This is similar to the cell saying, “Don’t come near me, I’ve been infected.” Hopefully, a roaming macrophage will consume the cell. It will present it to some lymphocytes in a lymph node and provoke an immune response.

There are two classes of MHC proteins. Class 1 MHC proteins are on all the cells of your body. Class 2 MHC proteins are only found on specific cells called antigen-presenting cells or APCs

Antigen Presenting Cells

Antigen presenting cells also called APC’s are cells that consume pathogens displaying foreign antigens. Antigen presenting cells do not need to know what the pathogen is in order to consume it. This means that antigen presenting cells are part of your innate immune system. The innate immune system is always functioning in the background.

Dendritic cells can be found, among other places, in the dermisThe thick inner layer of the skin that contains blood vessels, nerves, and connective tissue. of the skinThe body’s largest organ, providing protection and regulation..  Dendritic cells do pinocytosis or the drinking of the interstitial fluids. As a dendritic cell drinks the interstitial fluids in the dermis, they consume a pathogen. They are not certain what it is.  They take it to a lymph node. They say, “Hey, I found this. I don’t know what it is, but maybe we should be worried about it.”

Macrophages do exactly the same thing. They wander through tissues. They consume pathogens by breaking them down. Then they present them again to lymphocytes in lymph nodes. They say, “I found this. I don’t know what it is, but maybe we should be aware of it.” Macrophages are monocytes that have moved from the bloodstream into tissues and take up permanent residence in the tissue. For example, alveolar macrophagesImmune cells found in the alveoli that engulf and digest pathogens, debris, and dust particles. are found only in the alveoliMicroscopic air sacs in the lungs where gas exchange occurs between air and blood. of the lungs. Kuppfer cells are only in among the hepatocytesLiver cells responsible for detoxification, metabolism, and bile production. of the liverA large organ that produces bile, detoxifies blood, and stores nutrients..

B cells a type of lymphocyte are also antigen presenting cells. However, the way B cells present antigens is quite different. The steps taken for them to provoke an immune response are different. It is unlike the immune response that occurs from dendritic cells and macrophages. Using a B cell as an antigen presenting cell is discussed in the humoral immunity mini lecture.

MHC Class I

MHC Class I proteins are found on all the cells of your body.  As your cells make their proteins, some are displayed on the MHC Class I proteins, acting like an identification tag.  Lymphocytes constantly frisk your cells. T cells are looking for these MHC Class I proteins to display your self antigens.  If the MHC Class I protein displays your self-antigen, the T cell will leave the cell alone.  However, let’s say that your cells is highjacked by a virus.  The virus takes over your cell and starts pumping out its own proteins.  These proteins get displayed on the MHC Class I proteins.  A T cell notices this and says, “Hey, wait a minute. You don’t have the right name tag on.”  The T cell then would initiate a process of destruction.

MHC Class II

MHC Class II proteins are only found on antigen-presenting cells. These cells include three types mentioned in the previous slide: dendritic cells, macrophages, or B cells.  First, let’s be clear here.  These three types of cells are your cells. Just like any other cells in your body, these cells have your MHC Class I protein. It resides in their cell membranes. They display the right protein on it to confirm they are definitely part of your body.  However, these cells also have MHC Class II proteins inserted into their cell membranes.  These proteins display antigens that the antigen-presenting cell found.  Remember, those APCs are gobbling up foreign substances and breaking them down in phagocytosis.  The broken-down parts are then displayed on these MHC Class II proteins.  This is like these APCs saying, “Hey, I found this. I don’t know what it is. Maybe we should be worried?  You decide.”  So.  To whom are they displaying these broken down pathogen antigens?  To lymphocytes in lymph nodes.  Macrophages are constantly presenting the foreign antigens they find and asking lymphocytes if they want to mount an attack.