In the microscopic world, cells communicate with one another using proteins and carbohydrates on their surfaces. Each one of these can broadly be referred to as an antigen. Antigens exist on the surface of our own cells and pathogens (disease causing agents) like: viruses, bacteria, or fungi. Antigens are how cells interact with one another in the body. Through a relatively complicated process, our bodies and animals’ bodies produce specific antibodies that bind to antigens on invading pathogens. Once they bind to the pathogen, they either target it to be destroyed by cells of the immune system, or disable it from functioning. Through this process, antibodies provide protection against current infections and are also held in reserve for future ones. I go into more depth of how antibodies are produced, and what they truly are, later in the article. I intend to first address the limitations that surround antibody testing for disease surveillance — as this is quite relevant today amidst the COVID-19 coronavirus outbreak. Antibody testing involves drawing a blood sample from a patient and sending it to a lab. The lab tests the blood sample for levels of antibody thought to be specific to the infection of interest and reports this as a titer. This titer gives an idea whether antibodies are present and at what level they are circulating in the blood. There are limitations to antibody testing, especially for newer diseases and currently the WHO is recommending against antibody tests for patient care for COVID-19 until they are further tested and validated for this disease. Testing for antibodies can be an excellent, efficient, and useful tool; however, there are four primary limitations with this type of testing that I believe are driving the WHO’s recommendations.
Limitation 1: Having antibodies in your system doesn’t mean you’re infected.
If the body has an active infection from a virus, bacteria, or otherwise, it will produce antibodies against it. However, having antibodies does not inherently mean that a person was, or is, infected. The cellular machinery that produces antibodies will carry out this duty against both benign and aggressive foreign antigens. In fact, the pathogen does not necessarily even need to be alive, or infectious, in order to stimulate antibody production. This is the basis by which many vaccinations work. For instance, the Rabies vaccine that I use to vaccinate dogs, cats, and ferrets routinely is an inactivated virus. The virus, while not actively infecting the patient, is still stimulating the immune system to form antibodies to it! What an elegant way to provide protection without causing disease! Antibodies produced will then remain in reserve to fight off future infection before it can cause problems. Another example is allergy. At its root, an allergy is an overreaction of the immune system to a benign antigen (i.e. pollen). So, if one were to survey a population for antibodies to say juniper pollen (a common allergen in New Mexico where I live) they would likely reveal a significant portion of people that have developed antibodies to juniper. That does not mean that juniper pollen is infectious. Therefore, it does not imply that they ‘fought off’ a juniper pollen infection (which doesn’t exist) and in fact, it doesn’t even mean that they have allergy to it. Though, it does suggest they’ve contacted either pollen from juniper or something very similar at some point. Similarly, a positive antibody test to coronavirus does not necessarily mean that person cleared a coronavirus infection, at least not with the current information we have. We have no way of proving whether people with antibodies were actually exposed to living virus or instead to benign viral antigens that stimulated an immune response.
Limitation 2: Antibody testing at a single time-point is hard to interpret.
If a person or animal was infected by an illness and cleared the infection, they would be expected to have antibodies to it when their blood is tested. Antibodies do not develop instantaneously after a pathogen enters the body because the body has to go through a process before they are ready to go. This process can take several days to several weeks depending on the pathogen. If I collect blood from a dog right after they were exposed to a disease then they would be unlikely to test positive for antibodies. Likewise, if I tested too late, they may not have retained high levels or any detectable antibodies at all. Now, some diseases have established titers of antibodies that are accepted as indicative of infection, but in general, you must screen multiple consecutive antibody titers to prove an active infection. This is called an acute and convalescent titer. The acute titer is collected right away, and then the convalescent titer is collected in 2–4 weeks which allows the organism to produce antibodies. A four-fold increase in titer values is commonly accepted as a positive result and indicates infection. A person or animal generally retains some antibodies if they have cleared an infection. The coronavirus antibody studies on people in New York and Santa Clara may both indicate that more people have been infected without symptoms than we realized. However, a one-time antibody test cannot guarantee that individuals were actually infected because, as I discussed above, antibodies in circulation are not always representative of the complete immune response. More research on the protective role and specificity of antibody production by the body to the novel coronavirus undoubtedly needs to be done.
Limitation 3: Antibodies are specific, but not THAT specific.
Antibodies contribute to your body’s “memory” of disease. This immunity allows for the body to mount a stronger and faster immune response to a second exposure than a first, hopefully destroying a pathogen before it causes illness. This is part of the idea behind herd immunity, though the concept is a little more complicated than that. However, if an antibody produced to a specific protein can also bind to a similar, unrelated protein then problems can arise. This is called cross reaction and it occurs because the antibody “thinks” it is binding the protein it was made to, but instead is binding to a similar one. This can be a protein on the same organism it was intended to kill, or something else entirely. For example, Rheumatic Fever is thought to be a cross reaction of antibodies meant to bind group A Streptococci bacteria that are instead targeting muscle proteins or collagen in the heart, joints, etc. Cross reactivity can likewise throw a wrench into many antibody tests because it can be difficult to prove that the antibodies you are detecting are truly specific to the disease being screened for. For example, Leptospirosis is a bacterial disease in animals and people. There are multiple subtypes of Leptospirosis that may or may not cause disease in a person or animal. However, many of the different subtypes induce a cross reactivity that allow protection against the others. So, for instance if I screen a dog for Leptospira interrogans (a species of Leptospirosis) I may find antibodies in that patient to it even if they have never been exposed. This patient could have instead been exposed to a different subtype. Many different infectious agents have multiple subtypes, species, or subspecies that share similar enough antigens that they trigger cross reactivity.
Limitation 4: Having antibodies does not guarantee disease protection.
Antibody testing is not as simple as a positive or negative. Why is this? Well antibodies are produced generally within a couple weeks of acquiring infection depending on the organism. Remember, antibodies are formed through a relatively complicated process by your immune system which takes time. It can take several days to weeks before maximum production is achieved. Antibodies, regardless of number may not confer continued protection to a given disease depending on the nature of the disease. I know I have mentioned several times that having antibodies as a memory can help us fight future infections, but for some diseases what worked once won’t work again. For example, viruses, like Influenza, are difficult to form lasting protection against because they change their structure so rapidly. All of the antibodies our bodies produced to the flu last year may not be relevant for next year’s virus. The fact that we may retain antibodies to flu viruses of years past would complicate antibody testing for protection against the flu. Even if I detected large quantities of antibody to the flu virus in my blood, the virus that arises this year may not be stopped by it. Antibody tests for diseases can rarely be taken as a simple yes or no as it pertains to protection unless extensive research has been carried out on the disease-causing organism.
The four limitations that I’ve discussed are what I perceive to be the most relevant ones pertaining to COVID-19 surveillance, which is why I chose to discuss them first. However, for those of you hoping to understand antibodies more in depth we need to discuss what exactly an antibody is. In order to understand what an antibody is, we need to start from the beginning with a little immunology. The immune system is an incredibly complex cohort of cells, proteins, reactive oxygen species, and a variety of other compounds that serve to protect the body they inhabit. In the simplest form, there are two types of immunity that work in different ways to help the body. The innate, or inherent, immune system is essentially your body’s first line defense. It contains white blood cells, like neutrophils or macrophages (monocytes), that act to destroy invading pathogens of all shapes and sizes quickly. As part of their initial response, these cells recruit inflammatory molecules and also attempt to ingest the foreign invader.
The adaptive, or acquired, immune system is responsible for antibody production. This is an elegant system of cells and proteins that form a “memory” against particular pathogens. The primary cell types of this portion of the immune system are T cells and B cells. There are a few subsets of each; however, we don’t need to review them all here. This website has a decent summary of the multiple parts of the immune system. It is through collaboration between the innate and acquired immune system that antibodies can be formed.
It all begins when a pathogen (disease causing agent) enters the body. All cells have proteins and carbohydrates around their surface, as mentioned earlier, called antigens. If they are on your own cells, they serve to signal your immune system not to attack, or to aid in general physiologic functions. Cells are constantly traveling through the bloodstream and interacting with one another via these antigens. If a macrophage or monocyte encounters a foreign cell, it will realize that these receptors do not belong to a “friendly” cell and will begin to try to ingest them. Once ingested, the macrophage breaks apart the foreign invader and presents an antigen to a helper T cell. These T cells specifically carry antigen from the front line of the innate immune system to immune cells that wait in reserve, called B cells. The B cell receives the antigen and some other signals from the helper T cell and goes on to form two separate cell types; memory B cells and Plasma cells. A memory B cell serves as a bank of sorts. It keeps its antigens handy in case they are needed in the future. Plasma cells have massive protein production capability and once formed they begin to make antibodies, or proteins that are specific to the antigen that started this whole process. Antibodies are also called immunoglobulins.
There are many different types of immunoglobulin (Ig) such as IgG, IgA, IgM, IgD, or IgE and their various roles go far beyond the scope of this discussion. This article has a nice summary of these differences. Antibodies are incredibly important to long term survival for many organisms, as they provide a defense system against reinfection by the same disease. They allow our bodies to “remember” past infections or exposures and mount a quicker, more specific, response if it happens again. They are the basis for how our vaccines work, and can be a useful tool in monitoring populations for infection and herd immunity. However, for the reasons discussed above, they can produce significant difficulty in disease surveillance as the testing is not always representative of what is actually happening in a population, nor in an individual. Disease surveillance is never easy, add to that a new or emerging disease, and it is nearly impossible to say what the significance of an individual test is in the short term. I hope that this article provides a nice summary as to what an antibody is, and what difficulties I see with antibody testing in assessing exposure and infection status.