Pathology Student

When people talk about immunodeficiency states, they’re usually talking about secondary immunodeficiencies, like AIDS. The primary immune deficiencies really don’t get much press. Which is unfortunate, because although they are much less common than secondary immune deficiencies, they still occur, and it’s important to understand them for that reason alone. Plus, they are very testable – either on board exams, or on class exams.

Time is short, though, and you need to know the basic points for each one without having to wade through a lot of chapters in a textbook. So, without further ado, here is a short, bullet-pointed list of the main disorders, with particular emphasis on the part of the immune system that is affected, and the clinical manifestations of the disease.

X-linked agammaglobulinemia

• Pre-B cells can’t differentiate into B cells
• Patients have no immunoglobulin (Ig)
• X-linked (so seen only in males)
• Presents at 6 months of age (when maternal Ig runs out)
• Patients get recurrent bacterial infections
• Treatment: intravenous pooled human Ig

Common variable immunodeficiency

• Group of disorders characterized by defective antibody production
• Affects males and females equally
• Presents in teens or twenties
• Basis of Ig deficiency is variable (hence the name) and often unknown
• Patients more susceptible to infections, but also to autoimmune disorders and lymphoma!

Isolated IgA deficiency

• Most common of all primary immune deficiencies
• Cause is unknown
• Most patients asymptomatic
• Some patients get recurrent sinus/lung infections or diarrhea (IgA is the major Ig in mucosal secretions)
• Possible anaphylaxis following blood transfusion (patients have anti-IgA antibodies, and there is IgA in transfused blood!)
• Increased incidence of autoimmune disease   (who knows why)

Hyper-IgM syndrome

• Patients make normal (or even increased) amounts of IgM
• But can’t make IgG, IgA, or IgE!
• X-linked in most cases
• Patients also have a defect in cell-mediated immunity
• Patients have recurrent bacterial infections and infections with intracellular pathogens (e.g., Pneumocystis jiroveci)

DiGeorge syndrome

• Developmental malformation affecting 3rd and 4th pharyngeal pouches
• Thymus doesn’t develop well
• Patients don’t have enough T cells
• Infections: viral, fungal, intracellular pathogens
• Patients may also have parathyroid hypoplasia
• Treatment: thymus transplant!

Severe combined immunodeficiency

• Group of syndromes with both humoral and cell-mediated immune defects
• Patients get all kinds of infections
• Lots of very different genetic defects
• Half of cases are X-linked
• Treatment: bone marrow transplantation

The best way to remember these might be to make a little chart, with the diseases in one column, and subsequent columns for transmission (X-linked or not),  immunologic defect (e.g., no immunoglobulin production), and clinical features (e.g., infant with recurrent bacterial infections).

Photo credit: Wikimedia (http://commons.wikimedia.org/wiki/File:Antibody_je2.png) via cc license.

Systemic lupus erythematosus is one of a few diseases that have earned the name “the great imitator.” It is a chronic, systemic illness with many, many possible symptoms in many different organ systems, and widely varying disease courses in different patients. We’ll go through a short summary of lupus here and save the more detailed information for other posts.

The underlying cause of lupus is not known. To manifest the disease, you most likely need have two things: 1) a genetic predisposition, and 2) some environmental trigger (sun exposure, drugs, etc.). Whatever the underlying cause, patients with lupus all have autoantibodies against self antigens. All patients with lupus have anti-nuclear antibodies (antibodies against some part of the cell nucleus – like DNA, histones, RNA-bound proteins, or nucleoli). Some patients also have anti-RBC, anti-lymphocyte, anti-platelet and/or anti-phospholipid antibodies as well.

The autoantibodies cause damage by forming immune complexes with their corresponding antigens (in the case of anti-nuclear antibodies, there must be some type of cell destruction to expose the corresponding nuclear antigens!), and circulating around the body lodging in places they should not be (like glomeruli, skin, joints, the pericardium, etc.). Remember what type of hypersensitivity reaction this is? (See the bottom of the post for the answer. But try to remember before you look!)

Organ systems commonly affected (with common manifestations) are:
1. Renal system (wide variation in manifestations, from painless hematuria, to lupus nephritis, to end-stage renal failure. One histologic hallmark is membranous glomerulonephritis with a “wire-loop” appearance due to immune complex deposition.)
2. Skin (the classic manifestation is a malar, or “butterfly,” rash, but scaly patches, ulcers, and other skin lesions can occur)
3. Nervous system (headache, mood disorders, seizures, psychosis, focal neurologic deficits)
4. Musculoskeletal system (arthritis of small joints of hands, muscle pain)
5. Cardiovascular system (pericarditis, endocarditis (called Libman-Sacks endocarditis), atherosclerosis)
6. Hematopoietic system (anemia, thrombocytopenia, leukopenia, antiphospholipid antibody syndrome)

There are two main forms of lupus: discoid lupus and systemic lupus erythematosus. Patients with discoid lupus have only skin (not systemic) involvement, and they do not have anti-DS DNA antibodies (an antinuclear antibody frequently present in patients with systemic lupus). Systemic lupus erythematosus is just what the name says: a systemic form of the disease. It is the more common form, unfortunately.

The prognosis is difficult to predict for individual patients. Some patients have very few symptoms; rarely, the disease course is acute and rapid. Most patients suffer a relapsing and remitting course over a period of many years (the overall 10 year survival is 80%). Acute flare-ups can be treated with steroids – but as of now, there is no cure.

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The type of hypersensitivity reaction that is characterized by immune complex deposition is type III.

Photo credit: NIH (http://www.niehs.nih.gov/health/topics/conditions/lupus/index.cfm), under cc license.

Well, if you read last Friday’s post, you already know: it’s used for determining the amount of fetal blood that has backed up into the mom’s circulation. It’s usually done for the purpose of determining Rhogam dose. You need to make sure you give enough Rhogam to suppress the mom’s immune response. If there has been a little bleed, you give a little; if there has been a big bleed, you need to give more. Take a look at this chart if you want to know exact doses.

Here’s how it’s done:

1. Prepare blood smear from mom’s blood.

2. Expose blood smear to acid bath (this removes adult hemoglobin, which is acid-sensitive) but not fetal hemoglobin.

3. Stain smear (fetal cells appear pink; maternal cells look like “ghosts”). Take a look at this nice blood smear to see what this looks like.

4. Count lots of cells and report percentage of cells that are fetal (specifically: you count the number of fetal blood cells per 50 low power fields. If you see 5 cells per 50 low power fields, that’s equivalent to a 0.5 mL fetomaternal hemorrhage).

If you want to get really fancy, you can look for fetal blood cells using flow cytometry. Using a sample of mom’s blood, apply an anti-HbF (fetal hemoglobin) antibody, and then run the sample through the flow cytometer. In the little printout, look for cells that stain intensely with HbF: these are baby’s cells! A few of mom’s cells will have weak HbF staining – this is normal in adults.

Image credit: adamr.stone (http://www.flickr.com/photos/adamrstone/3098924060/) via cc license.

Hemolytic disease of the newborn (HDN) is a disease in which there is hemolysis in a newborn or fetus caused by blood-group incompatibility between mother and child.

There are a bunch of related terms:

• Immune hydrops (Hydrops means accumulation of edema fluid in the fetus during intrauterine growth. It is not specific to HDN, but can occur in many different fetal conditions including cardiovascular conditions, chromosomal disorders like Down syndrome, non-immune fetal anemia, twin-twin transfusion, infections, tumors, and metabolic disorders. Whew.)
• Hydrops fetalis (When the accumulation of fluid – from whatever cause – is severe and generalized, it is called hydrops fetalis.)
• Erythroblastosis fetalis (Erythroblastosis means that early red cell precursors are showing up in the peripheral blood. This can happen in any severe anemia, not just HDN.)

Mechanism of HDN

1. Fetus inherits blood group antigens (usually Rh D antigen or ABO antigens) from the father that are foreign to the mother.
2. Fetal blood gets into mom’s circulation (either during last trimester of pregnancy, when cytotrophoblast is no longer present, or during childbirth).
3. Mom makes antibodies to these blood group antigens.
4. Antibodies cross the placenta, attack baby’s red cells, causing hemolytic anemia and its consequences.

The consequences of hemolysis are numerous. One such consequence is extramedullary hematopoiesis. If the anemia is mild, extramedullary hematopoiesis in the liver and spleen may produce enough red cells to maintain normal numbers.

Other consequences are not so happy. If the anemia is severe, the heart and liver may suffer hypoxic injury, resulting in circulatory and hepatic failure. Liver failure causes decreased protein levels (proteins are synthesized in the liver) and a reduction in oncotic pressure in the circulation. Heart failure causes an increase in venous pressure (blood is backing up behind the failing heart). If severe enough, the combination of reduced oncotic pressure and increased venous pressure leads to generalized edema and ascites, a condition called hydrops fetalis, which can be fatal. Lesser degrees of edema can also occur.

If hemolysis is severe, jaundice can occur due to accumulation of unconjugated bilirubin. Unconjugated bilirubin is water insoluble; it binds to lipids in the brain (the blood-brain barrier in the fetus is poorly developed), causing serious damage to the CNS, termed kernicterus. The affected brain is enlarged, edematous, and yellow.

Rh-mediated HDN

Rh-mediated HDN most often involves the D antigen (sometimes it involves E or c; rarely it involves e or C). The baby inherits the D antigen from father, and mom is D negative (same as saying “Rh negative”). Fetal blood gets into mom’s circulation (through trauma, ruptures in the placenta during pregnancy, medical procedures carried out during pregnancy that breech the uterine wall, or childbirth). As a result, mom makes anti-D antibodies (the amount of antibody made depends on dose of antigen received from baby! Mom only makes anti-Rh antibodies when the she has received more than 0.5- 1 mL of Rh + cells.). Just like any other developing antibody, IgM appears first, and IgG appears later. This is important because IgG can cross the placenta, but IgM can’t. So HDN is uncommon in a first pregnancy. But if the mom gets pregnant again, and the fetus inherits D again, mom will now make IgG antibodies, and HDN can happen then.

Rh-mediated HDN is diagnosed using a direct antiglobulin test (DAT). This test will be positive in the baby (baby’s cells are coated with mom’s antibodies). An indirect antiglobulin test  will be positive in mother (though if the mother has received Rhogam at 28 weeks – keep reading – the IAT will be artificially positive!). Administration of anti-D antibody (Rhogam) at 28 weeks and again within 72 hours of delivery to Rh-negative moms prevents HDN in the current pregnancy (and, if mom has not produced anti-D yet, protects future pregnancies too) by coating any circulating D+ fetal red cells before mom is able to make any anti-D antibodies! The incidence of Rh-mediated HDN has gone way down since Rhogam was developed.

In order to determine the appropriate dose of Rhogam to give the mom, you have to quantify the amount of fetomaternal hemorrhage. This is done using either the Kleihauer-Betke test or an immunophenotyping assay. We’ll discuss these tests in our next post.

ABO-mediated HDN

ABO incompatibility occurs in 20-25% of pregnancies…but laboratory evidence of hemolytic disease occurs only in 1 of 10 such infants, and the hemolytic disease is severe enough to require treatment in only 1 in 200 cases.

There are a number of reasons why ABO incompatibility is rarely serious:

1. Most anti-A and anti-B antibodies are IgM (hence they don’t cross the placenta).
2. Neonatal RBCs express A and B poorly (the expression of A and B antigens increases as the baby grows).
3. Many cells other than red cells express A and B antigens and thus sop up some of the transferred antibody.

ABO hemolytic disease occurs almost exclusively in infants of A or B type born of group O mothers. Normal anti-A and anti-B antibodies are IgM and therefore don’t cross the placenta. For reasons not understood, however, some group O women have IgG anti-A and anti-B even without prior sensitization! In this situation, a firstborn child may be affected. Fortunately, even with transplacentally acquired antibodies, lysis of infant red cells is minimal.

ABO incompatibility is diagnosed with same tests as Rh incompatibility (DAT, IAT, Kleihauer-Betke test). There’s no effective protection against ABO incompatibility reactions! Good thing they’re not very common.

Treatment of HDN

It’s obviously way better to prevent HDN than to try treat it once it’s developed. That’s why the mother’s blood type is determined very early in pregnancy, and Rhogam is administered if mom is D negative.

If HDN does develop, there are several options for treatment. Minimally affected newborns can be treated with phototherapy (as in the photo above). Light oxidizes unconjugated bilirubin (toxic) to water-soluble, readily-excreted dipyrroles (harmless). Severely affected fetuses can be treated by total exchange transfusion of the infant (through umbilical vein). The mother can be treated with plasmapheresis (which removes antibody). High-dose intravenous immunoglobulin can be used too – but the best dosage and timing are not well-defined.

Photo credit:  treehouse1977 (http://www.flickr.com/photos/treehouse1977/3310309612/in/photostream/), under cc license.

The MHC (major histocompatibility) complex is a collection of genes on chromosome 6. It’s organized into three regions: the class I region, the class II region, and the class III region. Class I genes encode glycoproteins expressed on the surface of nearly every nucleated cell in the body. These glycoproteins, called MHC I receptors, present antigens (proteins made within that particular cell) to CD8+ T cells. Class II genes encode glycoproteins expressed only on the surface of antigen-presenting cells (macrophages and dendritic cells). These glycoproteins, called MHC II receptors, present antigens (which that antigen-presenting cell has eaten and processed) to CD4+ T cells. The MHC III region encodes a bunch of things, including complement proteins and cytokines.

So what?

Well, your T cells can’t recognize antigen unless it’s shown to them by an MHC receptor (the books say “presented in the context of an MHC receptor” but I like simpler words better). Helper (CD4 +) T cells recognize antigens only when they are presented by MHC II receptors; cytotoxic (CD8 +) T cells recongize antigens only when they are presented by MHC I receptors.

How are you supposed to remember which cells have MHC I receptors and which have MHC II receptors, and which T cell subset recognizes each one? I think of it like this. The MHC I receptor is kind of a low-class, ordinary, run-of-the-mill receptor (hence the common-sounding “I” designation). It’s present on virtually all nucleated cells in the body. The MHC II receptor is a high-class, specialized, advanced kind of receptor (hence the higher “II” designation). Only certain kinds of cells (antigen presenting cells) get to have this receptor. I’m sure that’s not why they named them that – but it helps me remember the numbers if I think of them in this way.

As to the cells that recognize each type of receptor – if you can remember the above, then you can figure out which type of T cell will recognize each receptor. Cytotoxic T cells recognize that type of MHC that is present on all cells in the body: the type I MHC receptor. It has to be this way, because any cell in the body can become infected, and cytotoxic T cells need to have a way to recognize and kill these cells. Helper T cells, on the other hand, recognize only that type of MHC that is present on antigen presenting cells: the MHC II receptor. Which makes sense, because helper T cells need the cytokines released by antigen presenting cells to help them do their job. So they lock onto the MHC II receptor on antigen presenting cells, and while they’re there, they get nice cytokine signals telling them to grow and differentiate.

Image credit: NIH.

Sometimes we (okay, I) get so caught up in describing pathologic mechanisms that real-life examples get the short end of the stick. Let’s look at some real diseases in which the underlying problem is a hypersensitivity reaction.

Type I (allergic) hypersensitivity
The big example (obviously) of this type of hypersensitivity is allergy. Pollen, cat dander, peanuts – they all have the same mechanism and this is it.

Type II  (antibody-mediated) hypersensitivity
There are a ton of diseases that have an underlying type II hypersensitivity reaction going on. Here’s a partial list:

1. Autoimmune hemolytic anemia: the patient makes antibodies to red cell antigens for some reason (not a good thing to do) which end up causing hemolysis.
2. Pemphigus vulgaris: the patient makes antibodies against the proteins connecting epithelial cells together, resulting in epithelial cell discohesion and bullae formation.
3. Goodpasture syndrome: the patient makes antibodies that react against proteins in both the glomeruli and the alveoli, leading to nephritis and lung hemorrhage.
4. Myasthenia gravis: the patient makes antibodies that bind to the acetylcholine receptor (on the muscle end plate), preventing acetylcholine from binding and doing its job; the end result is muscle weakness.
5. Graves disease: the patient makes antibodies that bind to the TSH (thyroid-stimulating hormone) receptor on thyroid epithelial cells, causing excessive stimulation of the receptor (just the opposite of what happens in myasthenia gravis!), leading to excessive production of thyroid hormone (hyperthyroidism).

Type III (immune-complex-mediated) hypersensitivity
There are a ton of diseases in this category too.

1. Lupus: the patient makes antibodies that bind to certain nuclear antigens; complexes lodge anywhere they please but especially in the kidneys, skin, and joints.
2. Post-streptococcal glomerulonephritis: in fighting a strep infection, the patient makes an antibody that reacts against the strep bug but also cross-reacts with some antigen in the glomerulus; antigen-antibody complexes lodge there and cause nephritis.
3. Serum sickness: after injection of foreign proteins into the patient (e.g., like they did in the old days when they used horse serum in vaccines!), the patient makes antibodies against the foreign proteins, and the resulting complexes lodge anywhere they want, but especially in the joints, kidneys, and vessels.
4. Arthus reaction: after injection of foreign protein into the skin, the patient (the original patient in Arthus’ 1903 experiments was a poor little bunny – but the same thing happens in humans) makes antibodies against this protein, and the resultant complexes stay in the skin, eliciting a nasty localized vasculitis and a big owie on the skin.

Type IV (T-cell-mediated) hypersensitivity
There are two kinds of this one (after all, there are two kinds of T cells!).

Delayed-type hypersensitivity
A good example of this is poison ivy exposure (patient gets exposed to poison ivy, helper T cells respond and some become memory cells; upon repeat exposure the memory T cells rush to the site, activating macrophages and causing inflammation). Another example is the Mantoux test; same principle.

T-cell-mediated cytotoxicity
Type I diabetes is a good example of this one. In this disease, cytotoxic T cells kill pancreatic islet cells. That’s not very nice. They are supposed to be killing infected cells, or tumor cells – not the patient’s own normal cells.

Photo credit: Joe Shlabotnik (http://www.flickr.com/photos/joeshlabotnik/506346418/in/photostream/).

Here’s a summary of those four pesky hypersensitivity reactions you will definitely be asked questions on at some point.

Sometimes, the best way to remember things is to boil them down to as few words as possible. I think you can summarize each hypersensitivity reaction in a one or two words:

Type I – allergy
Type II – antibodies
Type III – immune complex
Type IV – T cells

Here’s a little more information:

Type I hypersensitivity is the mechanism underlying the classic allergic response. It’s also called “immediate” hypersensitivity, which makes sense to any allergy sufferer (as soon as you start petting the cat, you start sneezing). It’s caused by an antigen (from an allergen, like cat dander) binding to IgE antibodies that are bound to the surface of mast cells. The antigen bridges the IgE antibodies, triggering release of nasty mediators (like histamine) from the mast cell. The end result: vessels dilate, smooth muscle contracts, and inflammation comes in and makes itself at home.

Type II hypersenstivity is also called “antibody-mediated” hypersensitivity. Which is kind of misleading, becase it’s not the only type of hypersensitivity reaction that involves antibodies. Oh well. In this type of hypersensitivity antibodies bind to antigens on a cell surface (any cell surface). Macrophages come in and eat up the cells (they think the Fc fragments of antibodies are yummy). Complement gets activated, inflammation comes in (harming tissue) and cells end up dying. Examples of this type of hypersensitivity include: autoimmune hemolytic anemia, pemphigus vulgaris, Goodpasture syndrome, myasthenia gravis, and Graves disease.

Type III hypersensitivity is also called “immune-complex-mediated” hypersensitivity. In this one, antibodies bind to antigens, forming complexes. These antigen-antibody complexes circulate (either throughout the whole body, or within one area of the body), get stuck in vessels, and stimulate inflammation, the end result being inflammation-mediated tissue damage and necrotizing vasculitis. Examples of this type of hypersensitivity include: systemic lupus erythematosus, post-streptococcal glomerulonephritis, polyarteritis nodosa, serum sickness, and the Arthus reaction.

Type IV hypersensitivity is also called “T-cell-mediated” hypersensitivity. This type of hypersensitivity has two subtypes. In one subtype, called delayed-type hypersensitivity, helper T cells secrete cytokines that activate macrophages (which eat the antigen) and induce inflammation (which damages tissue). A good example of delayed-type hypersensitivity is poison ivy. The other subtype, called T-cell-mediated cytotoxicity, involves cytotoxic T cells coming and killing target cells (like the cells of a transplanted organ, or the pancreatic islet cells in a patient with type I diabetes).

The nice kitty image is from anomalous 4 at: http://www.flickr.com/photos/31333486@N00/2066349843/

Here’s a nice boards – type question that requires you to put together some clinical and laboratory data to form a diagnosis, and then describe what the blood smear would look like.

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A previously-healthy 32-year-old woman presents with recurrent headaches. She develops nausea, vomiting, weakness, and easy bruisability. Her laboratory values are as follows:

Hgb = 6.9
MCV = 80
Plt = 30,000
WBC=15,000

She is immediately admitted to the hospital. Over a two day period, she becomes lethargic, and lapses into a coma. A peripheral blood smear is most likely to show which of the following?

A. Macrocytes
B. Spherocytes
C. Schistocytes
D. Sickle cells
E. Target cells

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This question describes a patient with marked anemia and thrombocytopenia (but the WBC is normal – even above normal – so you can rule out the causes of pancytopenia). Her clinical symptoms include various non-specific findings, like nausea and vomiting, but also the specific finding of neurologic deficits.

Her symptoms and laboratory values are consistent with TTP (thrombotic thrombocytopenic purpura), which is a disorder that’s part of a group of disorders called “thrombotic microangiopathies.” Thrombotic microangiopathies are a group of syndromes, including TTP and HUS, in which something triggers platelet activation, and the patient ends up with thrombi all over the place – with resultant thrombocytopenia (the patient is using the platelets up in making the thrombi) and microangiopathic hemolytic anemia (red cells are getting snagged on the fibrin strands as they go through the thrombi-laden vessels).

In TTP, the “something” that triggers platelet activation is abnormal von Willebrand factor. It turns out that normally, when von Willebrand factor is released into the circulation, it’s in a big, long multimer (called “unusually large vWF.” In this state, it causes platelet aggregation! But we have an enzyme called ADAMTS13 that cleaves unusually large vWF into smaller, less active pieces, which work well (but not too well).

Patients with TTP lack this enzyme (either they’re making antibodies to it, or they are missing it from birth) – so they have this unusually large vWF floating around, which traps platelets and causes thrombi.

The symptoms in TTP are usually described as a pentad: 1) hematuria and jaundice (from the microangiopathic hemolytic anemia), 2) bleeding and bruising (from the thrombocytopenia), 3) fever (who knows why), 4) bizarre behavior or neurologic deficits (from thrombi in the brain), and 5) decreased urine output (from thrombi in the kidneys leading to kidney failure). In reality, not all patients have all of the symptoms – so you don’t need to have all 5 by any means to diagnose it. Treatment depends on whether it’s acquired or hereditary. Acquired cases tend to be more severe for some reason, and the treatment involves daily plasmapheresis (to get rid of the antibodies to ADAMTS13). The hereditary cases are usually treated with plasma infusions every three weeks or so (to replace the missing enzyme).

So: the answer to the question is C, schistocytes. Schistocytes are a special type of fragmented red cells (they are small, with pointy outlines, and no central pallor) that are characteristic of microangiopathic hemolytic anemia (you can see a whole bunch of these in the image above!). Sometimes you can see things that look sort of similar to schistocytes in other anemias – such as the bite cells you see in glucose 6 phosphate dehydrogenase deficiency. But any time you see something you even think might be a schistocyte, you need to explore the possibility of a microangiopathic hemolytic process, because some of the things that cause microangiopathic hemolytic anemia are very dangerous (like TTP!), and you would not want to miss them.

Note: the very nice image of a microangiopathic hemolytic anemia (see all the schistocytes?) was taken by Ed Uthman and can be found at http://commons.wikimedia.org/wiki/File:DIC_With_Microangiopathic_Hemolytic_Anemia_(301920983).jpg.

We’ve been talking a lot about hemolytic anemias – we talked about how to figure out if your patient has a hemolytic anemia, and we talked about the DAT as a test that you would do to determine whether your patient’s hemolytic anemia falls into the immune category (warm or cold autoimmune hemolytic anemia) or the non-immune category.

Let’s look at the causes of hemolytic anemia, in general, and try to make sense out of a big list of disorders. We’ll go into depth on each disorder in separate posts.

One good way to think about the causes of hemolytic anemia is to break them down into inherited and acquired causes. The inherited causes of hemolytic anemia usually have some kind of defect in the red cell itself. Some involve  defects in the red cell membrane (like hereditary spherocytosis and hereditary elliptocytosis) such that the membrane becomes unstable; some involve enzyme deficiencies (like glucose 6 phosphate dehydrogenase deficiency, where the patient is missing an enzyme that detoxifies the red cell); and some involve defects in globin structure or synthesis (like the hemoglobinopathies, in which there are qualitative defects in hemoglobin, or the thalassemias, in which there are quantitative defects in hemoglobin).

Acquired causes of hemolytic anemia can be immune-related (we already talked about warm and cold autoimmune hemolytic anemia), infection-related (e.g., malaria or clostridium infection), drug related (tons of drugs can elicit hemolysis), or related to something outside the red cell that is ripping cells up (these are called microangiopathic hemolytic anemias, and there are tons of different “somethings” that can rip up red cells).

Usually, the inherited hemolytic anemias are chronic in nature. The patient often has a mild anemia (or no anemia) most of the time, because the bone marrow is working at its maximum to produce more red cells to replace the ones that are being destroyed. But sometimes, these patients undergo “crises,” in which something precipitates an increase in red cell destruction (parvovirus B19, for example, which loves to attack and destroy red cells) – and the already-maxed-out bone marrow can’t compensate, leaving the patient with way less red cells than normal. Acquired hemolytic anemias are usually acute in nature. Something happens to precipitate the anemia, and it’s a one-time, big hit to the red cells.

There are different ways to work up these anemias, but one thing that is done right away is the DAT. That will tell you whether your patient’s anemia falls into the immune category or not. If the DAT is positive, you have an immune process. If it’s negative, then you have a non-immune process – and we’ll talk about how to separate out all the non-immune causes of hemolysis when we discuss each disorder.

Note: the nice image of the red cell above was rendered by Andrew Mason, and can be found at: http://www.flickr.com/photos/a_mason/19191446/.

We talked recently about the direct antiglobulin test (DAT) which is a test used to find out whether a hemolytic anemia is immune-related or not. And we talked even more recently about the immune hemolytic anemias, starting off with warm autoimmune hemolytic anemia. As you now know, immune hemolytic amemias (also called autoimmune hemolytic anemias) come in two kinds: warm and cold. They are named that way because the antibodies in each type react best at different temperatures. That sounds like one of those useless, arcane facts likely to show up on a test somewhere. It may very well show up on a test somewhere (in which case, you’ll nail it), but it’s not arcane or useless, surprisingly; it has real relevance to what’s going on in the patient, as we’ll see in this post.

There are a couple things going on in cold autoimmune hemolytic anemia (AIHA): 1) the patient is making antibodies against his/her red cells, and 2) there is complement fixed to the patient’s red cells. As in warm AIHA, the patient is for some reason making antibodies to his or her red cells. The possible causes are numerous and include infections (like mycoplasma pneumoniae or infectious mononucleosis), lymphoproliferative disorders (like leukemia or lymphoma), and the good old “idiopathic” category (or, We Don’t Know What Is Causing It But We Don’t Want To Sound Silly). These antibodies are different than those in warm AIHA, though: they are IgM in nature (in warm AIHA they’re IgG), and they bind best at cold temperatures, like 4 degrees C or 39.2 degrees F (in warm AIHA, the antibodies bind best at warmer temperatures).

You may be thinking, yeah, so? Well, here’s the cool thing: because the antibodies bind best at cooler temperatures, they seem to bind to the red cells in distal parts of the body like the fingertips and earlobes (especially if the patient goes out in the cold), and they fall off in warmer, more central parts of the body. What’s more, because the antibodies are IgM in nature (remember, IgM is a pentamer), they are able to span several red cells, creating big agglutinates of red cells (you can see them in the image above). These agglutinates can plug up little vessels, creating ischemic conditions in these peripheral body parts. Which translates clinically into colorless (or blue), numb fingertips/earlobes/nose tips/whatever else is hanging out in the cold. Then, as the red cells circulate back into warmer, more central parts of the body, the IgM falls off, and the agglutination disappears.

So that’s the deal with the antibody. There’s also the matter of complement binding to the red cells. This is not good, because complement pokes holes in red cells! That’s where the hemolytic part comes in in cold AIHA – the red cells get busted open by complement, right in the circulation. It’s not like warm AIHA, where the hemolysis is happening mostly in the spleen; here it’s happening intravascularly. There is a little bit of macrophage action in the liver and spleen (macrophages see complement-coated red cells as yummy and eat them up) but mostly, the hemolysis is happening in the vessels.

Cold AIHA can hit any age, race, or sex. When it happens in elderly patients, it is usually related to some lymphoproliferative disorder; when it happens in kids, it’s usually related to an infection. The anemia is usually pretty mild, and the patient gets pallor (paleness) and/or cyanosis (blue discoloration due to lack of oxygen) in peripheral body parts, especially when exposed to the cold, because of those red cell agglutinates in tiny vessels.

When you look at a blood smear from a patient with cold AIHA, you’ll see big red cell agglutinates (but only if you have smart lab techs who cool the blood down first before making the smear, so you can see the agglutinates! Most lab techs are very smart.) You might see some spherocytes here and there too (because there is a little bit of macrophage nibbling going on) but they are way less numerous than they are in warm AIHA. To prove that your patient’s anemia is, in fact, an autoimmune hemolytic anemia, you need to do a DAT. The DAT will be positive in cold AIHA because the DAT looks for both IgG and complement bound to the patient’s red cells. Good thing they include the complement in the test (if they just included IgG, then cold AIHA cases would have a negative DAT).

The treatment for cold AIHA is what you’d expect: get rid of the underlying cause, if there is one (i.e., treat the infection), and keep the patient warm! The pallor/cyanosis in distal body parts is usually not severe enough to cause infarction – but it is annoying,  and patients should be kept warm so they don’t have to suffer from those symptoms, on top of everything else they are dealing with.