[main source: Cellular and Molecular Immunology by Abbas and Lichtman; this is my first pass over this material, so any corrections are especially appreciated]…
The Immune System starts at the Borders of the body… Skin, Respiratory System, and Gastrointenstinal Tract. These areas are lined with Epithelial Cells, and embedded in these are copious amounts of Dendritic Cells. Dendrites are monitor-reporters. Dendrites are on the look-out for invading Pathogens, and when they find them, they ingest and internally dismantle them. Then, the Dendrites display the Pathogens’ broken pieces on their surfaces.
The Dendrites then head toward a Lymph Node. The body’s Lymph Nodes are like intersections where Dendrites displaying Pathogen-pieces can run into Lymphocytes, circulating cells of the Immune System which also frequent the Lymph Nodes. Lymphocytes, collectively, form the main engine of the body’s Immune System response to Pathogens. Dendrites are merely the sentries are the Immune System; they cannot, by themselves, take on an invading army of baddies.
Lymphocytes come in two main types… T-Cells and B-cells. The B-cell variety is produced in Germinal Centers and come in an enormous variety of different subtypes (millions of different subtypes!), and each subtype is differently reactive with other cells it meets in the body. Anything with which a B-Cell Lymphocyte reacts that triggers the Lymphocyte to began a immuno-response is called an Antigen. Antigens are assumed by the Immune System to be represent treasonous activity in the body since they are usually components of Pathogens.
Getting back to our travelling monitor-reporters… the Dendrites (the digested pieces of their victims in full display on their surfaces) have come to the Lymph Nodes of the body where they hope to meet-up with the specific subtype of B-Cell made to react with the very pieces of Pathogen (Antigen) they are displaying. When the infection of Pathogens is serious enough, enough Dendrites will migrate to the Lymph Nodes to cause a chain of reactions which will actually swell the Lymph Nodes. That’s why swollen Nodes are a sign of infection.
When the right B-Cell meets the right Antigen-Presenting Dendrite– sparks fly (well, not literally)…
Where there is one Antigen, there is likely thousands or millions more with which to contend, so to improve the odds of the coming fight with Pathogens, the B-Cells begin cloning themselves– so that their combined reactions will produce a substantial, hopefully Pathogen-overwhelming, immuno-response.
The B-cells will then start producing a substance– specifically designed to interact with the Antigen they have been shown by the Dendrites. This substance is called Antibody. The recipe of Antibody varies depending on the type of Antigen the B-cells have been presented.
Antibodies are glycoproteins called ImmunoGlobins. Although there are other ImmunoGlobins in the body (we’ll speak specifically of IgE shortly), Antibodies are made by B-cells and ONLY made by B-Cells.
After being released by B-Cells, the Antibodies circulate until they come in contact with the Pathogen which contains the specific Antigen for which they were born to be attracted to. The Antibodies will then bind to that Pathogen.
The Antibody-Pathogen binding serves two purposes… 1) it keeps the Pathogen from binding somewhere else and causing it’s particular brand of trouble, and 2) it targets the Pathogen for Phagocyte consumption. Phagocytes have receptors for the tail-ends of Antibody (the ends NOT bound to the Pathogen). Phagocytes so thoroughly digest a Antibody-coated Pathogen that no part of the Pathogen is any longer a threat.
Simultaneously at the infection site, the other type of Lymphocyte, the T-Cells, have been attracted to the area by the set of chemical reactions unleashed by the Dendrites in their journey to the Lymph Nodes. With all the T-Cells and Pathogen-Antibody complexes now at the infection, the general area will begin to undergo inflammation. All sorts of T-Cells come to check out the scene, but only the helpful ones are preferentially selected to stick around, the rest eventually moving on to places where they can hopefully be more helpful– or at least, less in the way. T-Cells assist Phagocytes (in ways that are currently beyond my knowledge to speak about).
If the inflammation part of the process goes on too long, damage can occur in the infected tissues, including Fibrosis, when normal tissue becomes replaced by Connective Tissues.
Some parasitic Pathogens are too big to be eaten by Phagocytes; these are coated with a substance called Cytokine which is secreted by certain kinds of T-Cells.
To coat an object with Cytokines or other material in order to promote Phagocytosis (the process of being eaten to death by Phagocytes) is to “opsonize” something.
When the Immuno-Army (Antibodies, T-Cells, Phagocytes, and other troops) arrives too late, the Pathogen may have already invaded one of the body’s cells. The only Pathogen-killing option left to the Immune System at that point is to destroy the cell which has been invaded. That’s right, the Immune System will target and destroy our OWN infected cells in what has been euphamistically termed, Cell-Mediated Response (the mediating “cell” being one of our own poor sacrificial cells).
Some bacteria have adapted to survive inside Pathogen-chomping Phagocytes. T-Cells are in charge of taking care of this situation; they recruit other Phagocytes to come put their infected brethren out of their misery.
When one of our own cells is infected, parts of the invading Pathogen will wind-up being displayed on the surface of the invaded cell. This allows the Immune System to recognize which cells are infected. Certain T-Cells, called Cytotoxic T-Lymphocytes (or CTLs) can release cytotoxic granules which will destroy infected cells, thereby releasing the contents of the cell into the open– including the Pathogen, which now becomes suddenly vulnerable to Antibody and Phagocyte attack.
After a successful battle with Pathogens, the Repair and Clean-Up crews arrive. For instance, Macrophages come and ingest any dead host-cell parts leftover as well as any cells which seem about to pop (apoptocize)– before they can release their innards and induce even more potentially damaging inflammation. Macrophages can divide at the site of inflammation, and can adapt to respond differently to the different stimuli they encounter.
This is also a time for After-Action Review, during which feedback mechanisms adjust responses up or down depending on the big-picture situation. For example, if the war is going well against this particular Pathogen, Lymphocyte (B and T Cell) proliferation may be slowed down via various negative feedback mechanisms.
Typically, if a certain Pathogen has attacked the body once, there is good chance it will try again some day. Fortunately for us, the first attack primes the body to respond even quicker and with more overwhelming force during any subsequent attacks. After an infection, Antibody-secreting Plasma Cells (altered B-Cells) migrate to the Bone Marrow along with what have been termed “Memory Cells.” The Plasma Cells will continue to secrete low levels of Antibodies for many years, but the Memory Cells will only be activated at the return of the specific Pathogen for which they have a “memory.”
The Immune System does a marvelous job… but it’s not without its frequent– sometimes deadly– malfunctions…
ALLERGIES — Allergies occur when the Immune System OVER-reacts to a perceived Antigen. Antigens causing allergic reactions are known as Allergins. Allergins can arrive through the Respiratory System (such as Hayfever), through the Skin (such as Poison Ivy), or through the Gut (such as peanut or gluten allergies).
SELF-ATTACK — There are several Auto-Immune Diseases in which the Immune System mistakenly attacks our body, including Arthritis and Multiple Sclerosis. Sometimes, certain chemicals can bind with our own body’s proteins and form a complex which acts AS IF it were an Antigen– and thus stimulates an Immune System attack; this is why some people are allergic to Latex. In the case of Cancer, the Immune System can be complicit when it is not producing an adequate response to runaway cell-proliferation.
BLOOD TRANSFUSIONS AND OTHER TRANSPLANTS — The body is not inclined to accept foreign matter– even when the invasion would be a beneficial one. Medical professionals have come along in way when it comes to convincing the body to accept transfused blood (now pretty well understood) and some tissue transplants… but there’s much work and learning still to be done in this field. Relatedly, some mothers’ bodies– without proper treatment- will reject their own fetuses due to immuno-responses against the father’s genetic material.