Immunity

Third and final post for this week's Topic Quiz!

Today we're going to touch on immunity. Emphasis on "touch on," because immunology is a whole field of its own.

Types of Immunity

There are two main types of immunity: innate and acquired. Innate immunity is basically all of the non-specific defenses that our body can use immediately. Examples of this include anatomical barriers such as skin, as well as non-specific molecules such as lysozymes and defensins. Acquired immunity, on the other hand, takes longer to kick in, but it can target and kill specific pathogens. There are two subtypes of acquired immunity: active acquired immunity, in which you build up antibodies to an antigen that you have been exposed to, and passive acquired immunity, in which you just steal antibodies from someone else (e.g. mothers pass antibodies to their babies, antiserum contains antibodies against specific antigens).

Antibodies

First up, we're going to talk about a very important aspect of acquired immunity: antibodies! Antibodies are produced by B-lymphocytes, which mature in the bone marrow. (An easy way to remember this is "B" for "bone.") Antibodies can hang out in the membrane of B-lymphocytes, serving as B-cell receptors (BCR). When B-cells are activated, they can differentiate into plasma cells, which continue to produce antibodies, or memory cells, which help the body "remember" the antigen for future reference.

Antibodies, as you're probably well aware, are Y-shaped structures. They are made up of four chains: two identical heavy chains and two identical light chains. The top "arms" of the Y are known as the F_ab end, which can bind to antigens. The bottom part of the Y is the F_c region, which can bind to F_c receptors on other cells.

Not all antibodies are created equal. There are five main types of antibodies:

• IgG- The most common type of antibody. They are able to cross the placenta.
• IgA- Antibodies present in secretions such as milk, saliva and bile. Generally seen as a dimer (i.e. they look like 2 antibodies stuck together).
• IgE- Antibodies prominent in allergic responses.
• IgM- Antibodies involved in the primary response to an antigen. Generally seen as a pentamer (look like 5 antibodies stuck together).
• IgD- function not quite clear right now.

Just a quick note on IgM and the primary response- during your first exposure to an antigen, it'll actually be IgM that kicks in first (well, before other antibodies that is). This is then followed by IgG. During subsequent exposures to the same antigen, IgG produces the main response.

Antibodies have many important functions. Firstly, as explained in my previous post, they can act as opsonins, which helps other cells to phagocytose the bad stuff. They can cause agglutination (clumping) of antigens and neutralise toxins, preventing them from entering cells. They can also play roles in activation of immune cells and complement. Antibodies are said to have a humoral response- that is, they act by travelling through blood.

T-cells

In contrast to the humoral response exerted by antibodies, T-cells exert a cell-mediated response. They mature in the thymus gland ("T" for "thymus!") and have T-cell receptors (TCR) in their membranes. T-cells are kinda fussy- most of them only interact with antigens that are presented to them on MHC proteins. So let's have a look at what MHC proteins are!

MHC proteins, or Major Histocompatibility Complex proteins, are membrane proteins present on all nucleated cells. They are also known as HLA, or Human Leukocyte Antigen. As well as presenting antigens, they are also important "self" markers: a large number of alleles code for MHC molecules so the chances of two people having the same MHC molecules is pretty much zero (unless they're identical twins).

There are two main classes of MHC molecules. MHC class I is present on all nucleated cells, and binds peptides from invading pathogens. This allows cytotoxic T-cells (yeah, there's different kinds of T-cells- I'll get to them in a bit) to kill the infected cell. MHC class II is only present on specialised antigen-presenting cells, such as macrophages and dendritic cells. MHC class II interacts with helper T-cells, which in turn activate B-cells and cytotoxic T-cells.

Now I'll finally get to the types of T-cells!  There are two main types: helper T-cells with CD4 markers (T_h) and cytotoxic T-cells with CD8 markers (T_c). Cytotoxic T-cells kill stuff whereas helper T-cells just help coordinate everything via cytokines. That's not to say they're not important: helper T-cells actually outnumber cytotoxic T-cells 2:1 because they're pretty damn important. Another type of T-cell that you should know about are suppressor T-cells, which turn off activated cytotoxic T-cells and thus prevent the immune response from going overboard.

NK cells

Another important type of lymphocyte to know about is the NK cell. NK (Natural Killer) cells are part of the innate immune system because they act quickly and non-specifically. They can bind to a range of microbial peptides, and are pretty good at killing cancer cells and virus-infected cells. One of their weapons of choice is interferon, which as I mentioned in my previous post, can interfere with viral replication and prepare cells for viral attacks.

Blood Groups

Unfortunately the immune system doesn't always produce desirable responses. One example of this is agglutination, or clumping, if you give someone the wrong type of blood in a transfusion.

As you probably know, people can be one of four blood types: A, B, AB and O. These letters refer to the antigens present on the surface of red blood cells. Even though red blood cells (a.k.a. erythrocytes) are non-nucleated and thus lack MHC antigens, they do have other glycoproteins on their cell surface. People with type A blood have A antigen, people with type B blood have B antigen, AB has both antigens, and O has neither. People also have antibodies against the antigens that they don't have- type A has anti-B antibodies, type B has anti-A antibodies, and so on. This is why type O (which has no antigens) is considered to be the "universal donor," whereas type AB is the "universal recipient."

Hypersensitivity (a.k.a. Allergies)

Another example of the immune response going awry is the allergic response. An allergy is basically just the immune system responding to something innocuous, like a peanut. There are four main types of hypersensitivity, but for now I'm just going to talk about Type I and Type IV. (These are pretty much the immediate and non-immediate drug reactions that I spoke about in a post for PHAR2210.) One thing that all types of hypersensitivity have in common, however, is that sensitisation (prior exposure to that antigen) is required first.

Type I hypersensitivity, or immediate hypersensitivity, occurs within minutes of exposure. Essentially, a sensitised person will develop IgE against the antigen, which sit pretty on mast cells. When the person is re-exposed, the antigen binds to the IgE, causing mast cells to degranulate and inflammation to occur.

Type IV hypersensitivity, or delayed hypersensitivity, takes a few more days to develop. This is mediated by T-lymphocytes. An example of a stimulus that can cause delayed hypersensitivity is poison ivy.

Fortunately, there are things that can be done to help allergy sufferers. Some people can get allergy shots, which are injections of small amounts of the antigen. This stimulates production of IgG. Upon subsequent exposure to antigen, this IgG will bind to the antigen, preventing its binding to IgE and degranulation of mast cells. This IgG doesn't stick around forever, unfortunately, so allergy sufferers do need to go back for allergy shots every once in a while.

And I'm done typing about the topics for the first test! Yay!

Inflammation

Second post for Pathophysiology!

Inflammation

Okay, you probably already have a vague idea of what inflammation is. When you have an inflamed insect bite or whatever, your skin becomes red, warm, swollen and sore. These are the main signs of inflammation.

So why does this actually happen? Well, the gist of it is that when an areas is inflamed, the vessels dilate and become more "leaky," allowing fluid and leukocytes (i.e. white blood cells) to leak out. The fluid and protein exudate can be serous (watery, like in burns), fibrinous (high fibrin, leaving scars) or purulent (containing pus, which is essentially just dead cells). The exudate leads to the swelling, the vasodilation and increased blood flow leads to redness and warmth, and if the swelling presses against nerves, pain can result.

Sometimes inflammation can also lead to systemic signs. These systemic signs include fever, a higher erythrocyte sedimentation rate (due to the agglutination of red blood cells), increased levels of C-reactive protein (CRP) (an opsonin involved in the acute phase of inflammation- don't worry, I'll explain opsonins in a bit) and leukocytosis. Leukocytosis is an increase in the number of WBCs, and the type of WBC that increases depends on the type of inflammation. In acute inflammation, the number of immature neutrophils increases; in chronic inflammation, the number of lymphocytes increases. (Don't worry, I'm going to explain the different types of leukocytes soon.) A differential count, which looks at the ratio between immature neutrophils and lymphocytes, can help in diagnosis. Allergies and parasitic infections may increase the number of eosinophils. In contrast, viral infections may lead to leukopenia (decreased number of WBCs), as some viruses destroy white blood cells.

Leukocytes

Let's backtrack a bit to talk about the main leukocytes involved in inflammation. Some important cells to know are mast cells, neutrophils, eosinophils, monocytes, macrophages and lymphocytes.

Neutrophils, the most common type of WBC, are the first cells to arrive at the scene. They are drawn to the site via chemotaxis, a process in which they move towards areas with a higher concentration of signalling molecules (which in this case includes bacterial peptides, complement, prostaglandins and so on). Once they get to the site in question, they attach to proteins called selectins that line the vascular wall in a process called margination, before eventually moving across the vessel membrane in a process called diapedesis.

Neutrophils have relatively short lives (24-48 hours), during which they engulf and digest pathogens (om nom nom!). Before they do this, though, they need to recognise their antigens so they know what to eat and what not to. One way in which they recognise antigens is by binding to opsonins, which are molecules that can be used to coat antigens. For example, when antibodies bind to antigens, the antibodies are opsonins. Antibodies are specific opsonins (that is, antibodies will only bind to specific molecules), but there are other non-specific opsonins as well, such as complement.

After neutrophils eat their prey, they digest it. This digestion occurs in phagosomes, where superoxide radicals help to eat things up. It does this by being converted into hydrogen peroxide and then into hypochlorite radicals, courtesy of the enzyme myeloperoxidase (which catalyses the latter step). Superoxide is originally created by the reaction of oxygen with NADPH in a process called an "oxidative burst."

Okay, enough about neutrophils! The next important cells to talk about are monocytes and macrophages, which are the next to arrive after the neutrophils. Monocytes circulate in the blood, but once they get into the tissue, they differentiate into macrophages. Macrophages, from macro (big) and phage (to eat) are big eaters! They are the main phagocytic cells of the immune system.

Finally, a few other cells! They're important too, but not so much for now.

Eosinophils are cells that mainly attack parasites, but unfortunately they attack allergens too. Rude.

Mast cells can secrete histamine-containing granules. Histamine is an important mediator of inflammation, as we'll see later.

Lymphocytes come in several types. The most important types to know for now are B-lymphocytes, which make antibodies, cytotoxic T-lymphocytes, that kill stuff directly, and helper T-lymphocytes, that release mediators and signalling molecules to help coordinate everything.

Chemical Mediators of Inflammation

Time to overwhelm you with names! I'm so sorry...

Histamine is released by mast cells and basophils in response to trauma, IgE binding to mast cells, complement and/or interleukin. It causes bronchoconstriction, vasodilation and increased vascular permeability. This is good in that it helps white blood cells get out into the tissue, but can be bad if it happens to excess. In excess, blood pressure can drop, causing anaphylaxis.

Complement is a collection of proteins made in the liver. The proteins have many different functions, and may help to increase vascular permeability, act as opsonins and so on. Some complement molecules can get together and assemble into a MAC, or Membrane Attack Complex. This is basically a pore that forms in the bacterial membrane, causing fluid to enter the bacteria, which in turn causes swelling and lysis.

Arachidonic acid, stored in membranes and released by phospholipase A₂ (which is activated by complement, which I guess highlights how important complement is), has a lot of important metabolites. Arachidonic acid can be broken down by lipoxygenase into the leukotrienes, or by cyclooxygenase into prostacyclins, prostaglandins or thromboxanes.

Leukotrienes are formed by the breakdown of arachidonic acid by lipoxygenase. They cause bronchospasm (bronchiolar constriction), vasodilation and increase vascular permeability, sorta like histamine. They also promote chemotaxis. Leukotrienes, unlike other arachidonic acid metabolites, are not affected by NSAIDs (non-steroidal anti-inflammatory drugs, like ibuprofen).

Prostaglandins, just like histamine and leukotrienes, promote vasodilation and oedema development. They also mediate fever and pain. Prostaglandins also have a few other functions in other areas of the body, but we don't have to go into those now.

Thromboxanes, like TXA2, are formed in platelets. These are a bit different to the other molecules we've discussed so far, as they actually promote vasoconstriction, as well as platelet aggregation.

Prostacyclins, like PGI2, are the opposite of thromboxanes. They are formed in endothelial cells and inhibit platelet aggregation while promoting vasodilation. This ensures that a clot somewhere in your body won't randomly spread somewhere else. Prostacyclins are less sensitive to NSAIDs than thromboxanes, which is useful: taking a small amount of an NSAID, like aspirin, can prevent clotting via thromboxanes, without affecting the prostacyclin pathway.

Cytokines are signalling molecules in the immune system which mediate interactions between WBCs and promote chemotaxis. They are produced by activated lymphocytes and macrophages. There are three main types that you need to know:

• Interleukins activate T-lymphocytes and increase vascular permeability. They are pyrogenic- that is, they induce fever.
• Tumour necrosis factor (TNF) causes release of proteolytic enzymes.
• Interferon interferes with viral replication and gets uninfected cells ready for viral attack.

And that's two out of three topics down before the topic test on Thursday! Yay!

Cell Injury

Hey everyone!

So for those of you who don't know where I've been, and are wondering why I haven't updated for so long, you are about to find out why! Basically, I'm currently on exchange in Canada. The units I'm taking are Pathophysiology, Immunology, Introduction to Teaching Music to Children, Beginning Language and Culture I (Inuktitut I) and Introduction to Statistics.

Because I'm busy with exchange and everything that comes with it (*cough*actuallyhavingtocookformyself*cough*) I probably won't update as much as I did back home. Also, not everything I say here will be relevant to the PATH and MICR units back home. Hopefully some of it will be though!

This first post is going to be about Pathophysiology, which I think is technically a third-year unit. One point I just wanted to mention is that over here, lectures are generally not recorded. Hence lots of people actually attend lectures. It's actually really weird to go to a pretty full lecture that's not a first-year lecture.

Anyway, time to move on to talking about the first topic!

Cell Functions Susceptible to Injury

The first topic is, as the title of this post suggests, cell injury. So let's look at the myriad of ways in which cells can be injured!

• Damage to the genetic apparatus (i.e. DNA). This can happen due to radiation etc. Nuclei can also break down- shrinkage of nuclei is called pyknosis whereas fragmentation is called karyorrhexis.
• Damage to the cell membrane. When cells are damaged, they release all of the stuff inside, like enzymes and so forth. These can be used as a diagnostic tool for finding out which areas of the body are damaged.
• Reduced capacity of the cell to produce ATP. One of the main effects of this is that the Na⁺/K⁺ pump no longer has enough energy to keep pumping out those ions. This causes more ions to hang around in the cell, which in turn causes more water to come in via osmosis, which eventually causes the cell to lyse (burst). One of the main causes of a cell losing its ATP-producing capacity is a lack of adequate blood flow.

Free Radicals

Free radicals are atoms that have an unpaired electron somewhere in this outer shell. This renders them highly reactive and liable to destroy stuff. Some common free radicals are hydroxyl radicals, which come from the breakdown of hydrogen peroxide (see, this is why you shouldn't drink bleach, kids), and superoxide (i.e. O₂ with an extra electron).

Free radicals can be formed by normal reactions in the body. For example, xanthine oxidase can break down hypoxanthine into uric acid and hydrogen peroxide, which in turn can break down into hydroxyl radicals. Exposure to radiation can also result in the formation of free radicals. Finally, Fe²⁺ can promote the formation of reactive oxygen-containing radicals.

Thankfully, we have several ways in which we can protect ourselves from free radicals. There are several enzymes tasked for this purpose, including but not limited to the following:

• Superoxide dismutase: converts superoxide and hydrogen ions into hydrogen peroxide and oxygen. Requires zinc to do so. And yup, I realise that the hydrogen peroxide is a problem, but it's not a problem for long due to the presence of...
• Catalase: Converts hydrogen peroxide into oxygen and water.
• Glutathione peroxidase: Converts hydrogen peroxide and glutathione into oxidised glutathione and water. Requires selenium.

Some molecules are also pretty good at "scavenging" for radicals and clearing them out of our system. These are known as antioxidants. Water-soluble vitamin C and lipid-soluble vitamin E are two examples of antioxidants.

Necrosis and Apoptosis

Necrosis is the death of a group of cells. There are several types of necrosis. Coagulative necrosis, which results from a lack of blood supply, causes proteins to denature but the cell outlines are still retained. Liquefactive necrosis is where cell contents are broken down by enzymes, but pockets of liquid remain. Finally, caseous necrosis, found in the lungs of tuberculosis-affected patients, is characterised by dead cells walled off by white cells (a.k.a. a granuloma).

While necrosis is the death of a group of cells due to injury, apoptosis is the controlled, programmed death of individual cells. In the process of apoptosis, internal proteases called caspases are activated. There is also a decrease in cytoskeleton formation, causing the cell surface to bleb. I've written about apoptosis on an earlier post, if you'd like a bit more detail.

Cell Adaptation

There are many ways in which cells adapt to changing environments. I'm only going to list off the really obvious, visible changes here.

• Atrophy- Decreased size of cells
• Hypertrophy- Increased size of cells
• Hyperplasia- Increased number of cells (but cells remain more or less the same size). This is in contrast to hypertrophy, which is the same number of cells but increased size.
• Metaplasia- Replacement of one cell type by another
• Dysplasia- Disordered arrangement, growth and nuclear shape. May be indicative of future cancer, and thus is one of the markers that they look for on Pap smears.
• Anaplasia- Undifferentiated cells. May be indicative of cancer.
• Neoplasia- Abnormal growth of new cells which are unresponsive to growth control. So basically just a fancy name for a tumour.

Tumours and Cancer

As mentioned above, a tumour, or neoplasia, is the abnormal growth of new cells which are unresponsive to growth control. There are two main types of tumours: benign and malignant. Benign tumours grow more slowly, have differentiated cells, and normally stay put. These are denoted by the suffix -oma. Malignant tumours, on the other hand, are undifferentiated, fast growing cells, which can spread to other areas of the body (metastasise). These are denoted by the suffixes -carcinoma and -sarcoma, and are generally what we refer to when we talk about cancer.

Cancer is a disease resulting from the accumulation of many DNA mutations acquired over time. Some of the most prominent genes involved include tumour suppression genes such as the retinoblastoma gene (it's not only involved in retinoblastomas, it's just that that's the first place where they found it) and oncogenes. Factors predisposing someone to cancer include poor DNA repair mechanisms and age (i.e. more time to acquire more mutations). Factors that can lead to cancer, including environmental factors like smoking, inherited factors like gene mutations and viruses like HPV, are known as initiators. Other factors that do not cause cancer but instead might help cancer cells grow, like oestrogen in breast cancer and testosterone in prostate cancer, are known as promoters.

Aside from the characteristics of malignant cancer cells discussed above, malignant cells are also able to avoid apoptosis by using telomerase to keep their telomeres long, VEGF (vascular endothelial growth factor) to grow new blood vessels for themselves, and lose their contact inhibition due to defective cadherin and catenin molecules (which normally make cells stop dividing once they're squished by other cells). Pretty cheeky I reckon.

Coagulation

Right, now onto the fun times of talking about that long coagulation pathway which you've probably seen somewhere before! Don't worry, you don't need to know about every single clotting factor. Just a few of them.

There are two main pathways that lead to coagulation: the intrinsic and the extrinsic pathways. The extrinsic pathway is thought to be more important than the intrinsic pathway when it comes to coagulation. The only thing you need to keep in mind for now is that part of the intrinsic pathway involves the use of Factor VIII and vWF (von Willebrand Factor) to create a platelet plug. Actually no, that's not the only thing. The other thing you need to keep in mind is that the two pathways meet at the step where Factor X is converted into Factor Xa, a step that requires calcium. Drink yo' milk, kids. (Huh, that's the second time I've said "kids" in this post. I must really like calling you a kid.)

What does Factor Xa do, you might ask? Well, it converts prothrombin into thrombin. Prothrombin is a substance that can only be created if you have vitamin K, but if you take warfarin (a blood thinner) then the synthesis of prothrombin is inhibited. Also, another substance called heparin can prevent prothrombin from becoming thrombin. That sounds like a bad thing, but y'know, having a disorder where your blood clots at inopportune times would suck too.

Anyway, we still have the rest of the pathway to look at! Thrombin catalyses the conversion of fibrinogen into fibrin (remember, if something ends with "-ogen," it's likely to be an inactive precursor of some other molecule). Fibrin, with the help of Factor XIII, can then form a clot. Yay!

But what if you don't want clots to form? Well then it's plasmin to the rescue! Plasmin is formed from plasminogen, with the help of tPA, or tissue plasminogen activator.

Coagulation time can be measured with a few tests. PT (prothrombin time) and INR (International Normalised Ratio) are used to measure the extrinsic pathway. PT is highly variable, so INR is used instead (INR is essentially just comparing your PT to that of a control sample). PTT, or partial thromboplastin time, is used to measure the intrinsic pathway. (Just remember- when measuring intrinsic, put an extra T "into" the acronym.)

Such diagnostic tests are important, as they can help diagnose coagulation disorders. Haemophilia A is one such disorder, in which Factor VIII is defective. Another common disorder is von Willebrand Disease (which as you might guess is a deficiency of von Willebrand Factor), which is treated with desmopressin, an analogue of vasopressin.

Wound Healing

Healing of wounds requires several steps- yup, above and beyond the coagulation pathway that I outlined above. Firstly, vasospasm, or contraction of blood vessels, occurs in order to prevent further blood loss. The coagulation cascade takes place, causing platelet plugs and clots to form. Neutrophils and macrophages come in and remove damaged cells.

Meanwhile, regeneration starts to take place. Nearby cells undergo mitosis, forming granulation tissue (essentially just the new stuff). Growth factors such as VEGF and FGF (fibroblast growth factor) promote angiogenesis. Finally, fibroblasts, if they have enough vitamin C, produce collagen fibres, which can shorten to produce a scar. This seals up the wound, but unfortunately collagen isn't quite an adequate substitute for the hair follicles, glands, nerves or whatever else was in that spot before the scar was formed.

And that's the end of my first post from Canada! Yay!