Re: How exactly do vaccines immunize you against a certain disease?

Emily,

You ask good questions. Lots of people have gotten vaccines. I am sure when you were small, your doctor gave you a tetanus, diphtheria and pertussis vaccine. This is a combined vaccine that has the antigens of tetanus toxoid (a toxoid is a chemically altered toxin that no longer has the ability to cause injury), diphtheria toxoid and pertussis toxoid. The bacterial toxins from which the toxoids were made are the cause of "Lock Jaw", respiratory paralysis and Whooping cough, respectively.

These toxoid vaccines were made by growing the toxin producing bacteria in a way so that they made lots of toxin. Then the toxin was "inactivated" with a chemical that cross-links and aggregates the protein so it no longer is toxic. It is still antigenic or more specifically immunogenic.

An antigen is something that can be "recognized" by an antibody. Your body makes antibodies (proteins that circulate in the blood and lymph which have structures that fit the shape of an antigen so well that they stick). Not all antigens are immunogenic. For something to be immunogenic, it has to have the characteristics that allow an injection (or ingestion, inhalation, etc) of it to trigger an immune response. There is a growing field related to understanding what makes antigens immunogenic and useful in vaccines. I have a web site that is somewhat advanced but might give you more useful and detaile information about what is important in designing vaccine immunogens.

When the immunogenic vaccine is injected under your skin, it interacts with cells in the skin and in the fluids found in the connective tissue under your skin. Some of these cells are intermediate forms of cells that will differentiate into "antigen presenting cells" and other cells might be cells specialized in taking up proteins and transporting them. In addition, the antigen by itself could flow into a vascular structure called a lymphatic.

Lymphatics flow in one direction. They usually start as tiny blind-ended lymph capillaries in the connective tissue of the skin or near the basement membranes of almost all the cells of the body. Fluid flows into lymphatics because the structure of their walls acts like a valve that allows inflow from the surrounding space but prevents outflow from the lymphatic lumen the other way. Cells and and fluid containg immunogens then flow toward lymph collecting points in organs called lymph nodes. Lymph nodes are found at various places of the body where your skin folds. You can find lymph nodes in your neck below the angle of your jaw, in your armpits and in your groin. You or your doctor probably felt them when they enlarged if you had "strep throat" or all of them enlarged if you had "mononucleosis."

Once in lymph nodes, immunogens will eventually associate with antigen presenting cells located in an area of the lymph node where millions of circulating lymphocytes travel from the blood to lymph. The process that brings these important cells of the immune system to lymph nodes and back to the blood is called "lymphocyte recirculation and homing." I have a web site that explains this process in more detail. You may want to visit it. I also have a web site that provides more detail about the "structure and organization of the lymphatic system," including an explanation how lymph nodes work to allow for efficient induction of immunity.

After the antigen is broken down by antigen presenting cells, the important "determinant" (a small part of the antigen that has the most unique amino acid sequence or 3D shape) is transported to the cell surface in proteins that belong to a group called "major histocompatibility antigens (MHC)." These were discovered because they were important antigens in triggering the rejection of foreign grafts, like skin or kidneys used to replace tissues that were lost or diseased. These MHC proteins, of which there are at least two types, carry the antigenic determinant in a pocket so that it sticks out at the same time as flanking molecules of the MHC protein also stick out. When a T-cell (a kind of lymphocyte that is involved in memory and triggering immune responses) binds to the antigen presenting cell it can only see the "foreign antigen" when it is presented this way. If it sees the antigen floating freely or stuck on some other kind of protein, the T-cell might not respond or might decide to shut down or even prevent an immune response to that antigen. If it sees the antigen in an MHC pocket, it triggers an immune response and the T-cell "Helps" that immune response get started in many ways. Two kind of immune response may start. A cellular response carried out by T-Cells (Helper Cells and Suppressor/Cytotoxic Cells) that is like armed combat. The lymphocytes have to touch a cell (like a cell infected with a virus) and kill it directly or indirectly. The other kind of response involves T- cells helping B-cells divide and differentiate into cells that manufacture huge amounts of antibody. Usually this antibody is secreted into tissue fluid and it finds its way into the blood via lymph. Then circulating antibody manufacture in one or many places in the body can attack foreign antigens anywhere. This makes B-cells making antibody like artillery or strategic missiles because the effect made locally has its benefit a long distance away.

You see. The simple process of injecting a vaccine into your shoulder, once or several times, results in your being protected from getting an infection or intoxication from a microbe by a rather elegant and complicated process and you don't have to know everything about it for it to take place. The body knows. The science of immunology was also fortunate to have come into existence because of this simple phenomenon of injecting something related to a disease-causing microbe that results in protection from ever having to suffer from the disease. It has been over 100 years since the first of these phenomenological observations, yet we are only just now able to see how it all works.

Thanks for asking such a neat question!