In microbiology we have two types of microbial diagnosis. The direct method is where we detect the invader's DNA, Antigens or culture to see the exact pathogen while the second, indirect, method is the reacting the antibodies of patient's serum with different reagents.

If we use the indirect method (antibody reaction) and find that they are producing antibodies against a specific disease (let's say streptococcus A) what is the benefit of having found that they are fighting Strep A since the body is already clearly fighting it and putting up the best defence to fight the disease?

Would a doctor really prescribe antibiotics even though here is already an immune reaction against the infection?

If diagnosis and treatment are useless then why do we waste the time to do it?


2 Answers 2


Short answer: Antibiotic treatment is important because the body can't always keep up with the infection even if it has launched a response.


First, let's look at what the antibody reaction method is actually doing.

  1. Taking a sample of blood or body fluid
  2. Testing the sample with known pathogens - viruses, bacteria, fungi, etc.
  3. Identifying which infection causes a reaction.

This allows us to determine which infection we are dealing with. This can be especially useful when the infection is hidden in a place we are unable to easily swab - the stomach, heart, a small wound that has already healed. Sometimes this test is just used to monitor how well you are fighting the infection, but it also allows them to evaluate what the best course of treatment is for you. (Reference 1)

Just because the body has an immune reaction doesn't mean that the body's immune system can actually finish the battle against a disease. In the example case of Strep A, you may be able to get over the infection yourself but it will take longer than if you are prescribed antibiotics against it.

See this question on the CDC:

Q: How common is invasive group A streptococcal disease?

A: Approximately 9,000-11,500 cases of invasive GAS (Group A Streptococcus) disease occur each year in the United States, resulting in 1,000-1,800 deaths annually.

  • Note that invasive Strep A usually infects children, elderly, and immunocompromised people (anyone with a weaker immune system typically).

Unless you have absolutely no immune system at all, there will be an antibody response put out by your body. The NHS goes into detail about different parts of your immune system.

Recall also that Strep A (our example) could divide faster than the body can keep up with. So we would need antibiotic help to fight the disease. Using the "indirect" method we can find a hidden infection.

According to the NHS the antibiotics put a stop to growth and spread which allows the body to come in and finally finish them all off, or they disrupt a process the bacteria needs to survive and kill the infection for the body. Thereby helping the immune system (the body's best line of defence) finish off the infection when it couldn't do it on its own.

  • $\begingroup$ It is also worth pointing out that the premise of the question is wrong. Assaying for an antibody response is rarely used to diagnose an ongoing disease, because the antibody response takes several days to develop; the exceptions are for chronic diseases (hepatitis B or C, HIV) where the pathogens have evolved defenses against the immune system and aren't cleared by it. $\endgroup$
    – iayork
    Commented Oct 9, 2015 at 17:26
  • $\begingroup$ @iayork aren't antibodies tested for various diseases? Lepto, dengue, typhoid just to name a few.... $\endgroup$
    – Polisetty
    Commented Oct 30, 2016 at 12:24
  • $\begingroup$ They're used to test whether you had the disease, sure. For acute diseases like typhoid and Dengue, they're less useful because they're not detectable for a week or so after the disease starts. $\endgroup$
    – iayork
    Commented Oct 30, 2016 at 13:35

One use of indirect testing is used to test for *exposure** to organisms that we wouldn't just naturally recover from as well. One example off the top of my head is HIV. The first line of testing for this infections is simply to look for antibody responses, but because it is not effectively resolved by the patient's immune response, it is important to know about this infection.

*Also note that because we are looking at antibodies, we only know that the patient has been exposed to the organism, not that they are actively infected. The antibody test is quick and easy though, so it saves money (and probably time) to do this first.

(originally, I included TB in this answer, however it is actually a skin test - not assayed ex vivo via ELISA, so I removed it.)


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