Your blood type is determined by specific molecules on your red blood cells that are encoded by your genes and inherited from your parents.

What actually makes a “blood type”?

When people say “What’s your blood type?”, they’re usually talking about two main systems working together: ABO and Rh.

1. The ABO system: A, B, AB, or O

On the surface of your red blood cells are tiny markers called antigens. In the ABO system, the important ones are A and B:

  • Type A: Has A antigens on red blood cells, and anti‑B antibodies in the plasma.
  • Type B: Has B antigens, and anti‑A antibodies in the plasma.
  • Type AB: Has both A and B antigens, and no anti‑A or anti‑B antibodies (universal recipient in ABO terms).
  • Type O: Has no A or B antigens, but has both anti‑A and anti‑B antibodies (universal donor in ABO terms).

These antigens are made by the ABO gene , which sits on chromosome 9. Different versions (alleles) of this gene tell your cells whether to put A, B, or neither of those antigens on your red blood cells.

A short, story‑style way to picture it:

Imagine your red blood cell as a soccer jersey.
The ABO gene is the “design code” that says whether you sew on an A patch, a B patch, both patches, or leave it blank.
That jersey design is what we call your ABO blood type.

2. The Rh system: positive or negative

On top of A or B, there’s another key molecule: the Rh factor , often specifically the RhD antigen.

  • If you have the RhD antigen: Your blood type is “positive” (A+, O+, etc.).
  • If you don’t have it: Your blood type is “negative” (A−, O−, etc.).

Rh factor is controlled by other genes (mainly on chromosome 1), and “positive” is generally dominant over “negative”.

So your complete blood type is a combo of ABO + Rh, giving the 8 familiar labels:

  • A+, A−, B+, B−, AB+, AB−, O+, O−.

3. How your parents decide your type (genetically)

You inherit your blood type like you inherit eye color: through pairs of gene versions from each parent.

For ABO:

  • Each parent has two ABO alleles (for example, A and O), but passes only one to you.
  • The main alleles are:
    • A
    • B
    • O (essentially “no A/B antigen”)
  • A and B are dominant over O, while A and B together give AB.

Typical patterns (simplified example):

  • AO + OO parents can have: A or O children.
  • AO + BO parents can have: A, B, AB, or O children.
  • OO + OO parents can have: only O children.

For Rh:

  • “Positive” tends to be dominant, so if you inherit at least one “positive” version, you are usually Rh+.

4. What’s happening in the blood itself?

A blood type is more than a label; it’s an immune compatibility code.

  • Antigens (A, B, RhD) sit on your red blood cells.
  • Antibodies in your plasma attack cells that have “foreign” antigens.

Example:

  • If you are type A, your immune system has anti‑B antibodies and will attack type B or AB blood as foreign.
  • If you are O, your plasma has both anti‑A and anti‑B, so giving you A, B, or AB blood can trigger a dangerous reaction.

That’s why matching blood type is critical for transfusions, pregnancy care, and organ donation.

5. Beyond ABO and Rh: many more blood group systems

When people talk casually, “blood type” means ABO and Rh, but scientifically, there are dozens of blood group systems.

  • More than 40 recognized blood group systems exist (like Kell, Duffy, Kidd), all based on different antigens on red cells.
  • These matter especially for patients who need lots of transfusions or have rare types, where even “small” antigen differences can trigger immune reactions.

Think of ABO and Rh as the big headline on your ID card, while all the other systems are the fine print that becomes important in special situations.

6. Why this is a “trending” topic lately

In the last few years, blood type keeps popping up in:

  • Public health news about blood shortages and urgent calls for O− or specific rare types.
  • Online discussions about whether blood type affects disease risk or severity (research continues, results are often subtle and not absolute).
  • Forums where people ask things like:
    • “Why are there different blood types at all?”
    • “Can my blood type change?”
    • “Can I guess my parents’ types from mine?”

Scientists are still exploring how different blood groups evolved and why certain types are more common in some regions than others.

Mini FAQ: Quick answers

What makes a blood type?

  • The specific antigens on your red blood cells (especially ABO and RhD).
  • The antibodies your immune system naturally makes against antigens you don’t have.
  • The genes you inherit from your parents that code for those antigens.

Can your blood type change?

  • Under normal circumstances, no. It is genetically determined and stays the same for life.
  • Rarely, bone marrow transplants or certain diseases can alter what appears on your red blood cells, effectively changing your tested type.

Why does blood type matter in hospitals?

  • To prevent dangerous immune reactions in transfusions.
  • To manage Rh incompatibility in pregnancy (for example, Rh− mother and Rh+ baby).
  • To match organs and blood for high‑risk or multi‑transfused patients.

Simple HTML table: main human blood types

html

<table>
  <thead>
    <tr>
      <th>Blood type</th>
      <th>ABO antigens on RBCs</th>
      <th>RhD antigen</th>
      <th>Common plasma antibodies</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>A+</td>
      <td>A</td>
      <td>Present (+)</td>
      <td>Anti‑B</td>
    </tr>
    <tr>
      <td>A−</td>
      <td>A</td>
      <td>Absent (−)</td>
      <td>Anti‑B</td>
    </tr>
    <tr>
      <td>B+</td>
      <td>B</td>
      <td>Present (+)</td>
      <td>Anti‑A</td>
    </tr>
    <tr>
      <td>B−</td>
      <td>B</td>
      <td>Absent (−)</td>
      <td>Anti‑A</td>
    </tr>
    <tr>
      <td>AB+</td>
      <td>A and B</td>
      <td>Present (+)</td>
      <td>None (to A/B)</td>
    </tr>
    <tr>
      <td>AB−</td>
      <td>A and B</td>
      <td>Absent (−)</td>
      <td>None (to A/B)</td>
    </tr>
    <tr>
      <td>O+</td>
      <td>None (no A/B)</td>
      <td>Present (+)</td>
      <td>Anti‑A and Anti‑B</td>
    </tr>
    <tr>
      <td>O−</td>
      <td>None (no A/B)</td>
      <td>Absent (−)</td>
      <td>Anti‑A and Anti‑B</td>
    </tr>
  </tbody>
</table>

TL;DR: What makes a blood type is the combination of antigens (like A, B, and RhD) on your red blood cells and antibodies in your plasma, all specified by the genes you inherit from your parents.

Information gathered from public forums or data available on the internet and portrayed here.