(NEXSTAR) – It’s widely known that the second dose of the COVID-19 vaccines tend to come with more side effects than the first, including tiredness, headaches, chills, fever, nausea and muscle pain.

Why is that?

According to Dr. William B. Greenough III, a professor emeritus at Johns Hopkins School of Medicine who’s known for his work on cholera, there’s a simple explanation for the increase in side effects.

With the first dose, your body begins building its initial immune response, including producing antibodies.

But with the second shot — a.k.a. the second exposure to the virus — “the big guns” of your immune system react.

“Once your immune system recognizes the virus [after the second dose], it’s going to have a greater reaction to it,” Greenough said.

This is a sign the vaccine is working and that your immune system “recognizes the vaccine in your body.”

With the second dose, your body not only creates antibodies, but spurs your lymph nodes and other parts of the body to react to the vaccine at the tissue level.

Why does your body create side effects at all? There’s a two-part answer to this question, according to Dr. Richard Kennedy, a professor of medicine at Mayo Clinic and the co-director of the Mayo Clinic Vaccine Research Group.

Your immune system has two different components — innate immunity and adaptive immunity. Innate immunity is your body’s first-line response to an outside invader. 

“Innate immunity does the same thing every time it sees a microorganism in your system. It’s got to recognize that you’re infected, sound the alarm and start the adaptive immune response,” Kennedy said.

“Innate immunity is not very good at protecting you against viral infections,” he continued. “It’s more of a speed bump to slow it down.” 

After your body’s innate response, which occurs at the first sign of an infection or virus, adaptive immunity takes over. This kind of immunity is responsible for the creation of B and T cells, which can blast away infection. 

When you receive a vaccine, your innate immunity is stimulated first. Within a week-and-a-half or so, your adaptive immunity starts firing, offering you actual protection from the virus. 

Frequently asked questions about the vaccine, answered:

How long should I wait to get the vaccine after having the virus?

According to Jenny Johnson, Public Information Officer with the Utah Department of Health, people who have had COVID-19 can safely be vaccinated.

The only “rule” about being vaccinated after being infected with the virus, she says, is that people must have completed the quarantine period and be symptom-free.

“There is no reason why someone should not get the vaccine after being infected,” Johnson says.

Can I donate blood after receiving the vaccine?

You can, but the American Red Cross says it is important to note which type of vaccine you got.

What should and shouldn’t I do after receiving the COVID-19 vaccine?

Do you continue to social distance and wear a mask? And when does immunity set in?

The Utah Department of Health provided ABC4 some guidelines.

I missed my second COVID-19 shot – now what?

The appointment is scheduled, and you missed getting it! What do you do now? Will you have to take two more shots? Probably not. Here’s what the Utah Department of Health says:

“You should get your second shot as close to the recommended 3-week or 1-month interval as possible. However, there is no maximum interval between the first and second doses for either vaccine. You should not get the second dose earlier than the recommended interval.”

When can children get the COVID-19 vaccine?

While the Pfizer vaccine has been approved for those 16 and 17-years-old, studies continue on the use of the Pfizer and Moderna vaccines on children ages 12 and older.

Do the vaccines have microchips in them?

No, the vaccines do not have a microchip in them. ABC4 spoke with a pair of experts who explain where the theory came from.

Can I take painkillers before or after receiving the vaccine?

It’s best to avoid them, unless you routinely take them for a medical condition, officials say. Although the evidence is limited, some painkillers might interfere with the very thing the vaccine is trying to do: generate a strong immune system response. Health officials explain why.

For continuing coverage of the COVID-19 pandemic and the vaccine, click here.