12.2 Blood Types, Rh, and Immunohematology Basics

AMT III.7.A.3 is one sentence that carries transfusion-safety weight: know that there are blood types, Rh types, and other antigens. Immunohematology applies antigen–antibody reactions to red cells. A person’s ABO and Rh type is not a chemistry panel and not a complete blood count. Issuing packed red blood cells is a regulated process (Food and Drug Administration blood-establishment rules, CLIA nonwaived testing, and AABB standards in accredited blood banks). A CMLA who can name types and tubes but who never independently “just hangs O-negative” is the assistant the blueprint describes.

Quick Answer: ABO types are A, B, AB, and O. Forward typing mixes patient red cells with reagent anti-A and anti-B. RhD present = Rh positive; absent = Rh negative. Other antigens (Kell, Duffy, Kidd, and more) also exist. The wrong unit can cause a fatal acute hemolytic transfusion reaction. A CMLA does not independently issue blood or perform anti-human globulin (AHG) crossmatches. Collect pink- or lavender-top EDTA, label at the bedside, and remember fibrinogen is a plasma protein, not an ABO antigen.

ABO blood types and the forward-typing idea

ABO antigens are inherited carbohydrates on the red-cell surface. People also have expected naturally occurring antibodies to the A or B antigens they lack—classically IgM, which can bind complement and lyse incompatible cells.

Forward typing (cell typing) tests the patient’s red cells with known reagent antibodies (anti-A, anti-B, and often anti-A,B). That is the “ABO forward idea” CMLA must recognize: you are asking what antigens sit on the cells, not what glucose sits in the plasma.

Reverse typing (serum or plasma typing) tests the patient’s plasma or serum with known A1 and B reagent cells. Forward and reverse results must agree. Reverse typing is not a waived cassette a CMLA freelances at the chair; it is part of ABO determination performed by qualified blood-bank personnel. If forward and reverse disagree, the workup stops until the discrepancy is resolved—no unit goes out on a guessed type.

Group O packed red cells lack A and B antigens, which is why emergency-release teaching calls O the red-cell “universal donor.” Group AB plasma lacks anti-A and anti-B. Emergency-release decisions are laboratory and medical-director protocols, not a CMLA independent action.

Rh types

The Rh system includes multiple antigens (D, C, c, E, e). RhD is the one CMLA must name. If D is present, the person is Rh positive; if D is absent, Rh negative. Unlike ABO, people do not naturally have anti-D. Anti-D forms after exposure: transfusion of D-positive cells to a D-negative recipient, or pregnancy with a D-positive fetus. That is why Rh-negative women of childbearing potential receive Rh immune globulin per obstetric protocol—not a unit a CMLA issues from a clinic refrigerator.

Weak D (historically “Du”) is a blood-bank interpretation. A CMLA does not independently assign weak D status or convert an Rh-negative type to Rh-positive because a cassette looked faint.

Other antigens

AMT’s wording is blood types, Rh types, and other antigens. Clinically important examples include Kell (K), Duffy (Fyª, Fyᵇ), Kidd (Jkª, Jkᵇ), MNS, and Lewis, among many others. Patients can make IgG antibodies to these after transfusion or pregnancy. Those unexpected antibodies are why a type and screen or type and crossmatch is more than ABO/Rh. You do not need every International Society of Blood Transfusion number. You do need to know other antigens exist and that an antibody screen looks for unexpected antibodies against them.

Why mix-ups are fatal

If a group A patient receives group B red cells, the patient’s anti-B binds donor B antigens. Complement can activate. Intravascular hemolysis follows: fever, chest or back pain, hemoglobinuria, hypotension, disseminated intravascular coagulation, renal failure, and death. This is an acute hemolytic transfusion reaction (AHTR), classically ABO incompatibility, and it is a sentinel event. Most ABO fatalities are clerical: the wrong patient was drawn, a tube was relabeled, or a unit was hung on the wrong patient—not a mysterious reagent failure.

CMLA prevention is identification and labeling, not clever serology:

• Identify the patient with two unique identifiers at the chair (CLSI PRE01).
• Label blood-bank tubes at the bedside from the armband. Never prelabel at the desk.
• Many laboratories require the collector’s signature or identification on the specimen, a dedicated blood-bank armband, and a redraw if any identifier is missing or disagrees.
• Do not add a missing identifier later from memory.
• Do not deliver an unlabeled pink-top “because they are the only patient in the room.”
• Document recent transfusion or pregnancy when the order asks; the blood bank needs that history for antibody workups.

Why CMLA does not independently issue blood or perform AHG crossmatches

An antibody screen mixes patient plasma with screening cells that carry common antigens, then uses anti-human globulin (AHG, Coombs reagent) to detect IgG that bound those cells in vitro—the indirect antiglobulin test (IAT). A crossmatch tests donor cells against patient plasma. An AHG (full, Coombs-phase) crossmatch is required when the screen is positive or when policy requires it; it detects IgG incompatibility that immediate-spin saline testing can miss.

These methods are nonwaived (moderate or high complexity). Mixed-field agglutination, warm autoantibodies, and antibody-identification panels are scientist/technologist interpretations. A CMLA:

• May collect, label, centrifuge if standard operating procedure (SOP) allows, and transport type-and-screen specimens.
• May perform only immunohematology procedures that are waived and written into the assistant’s facility SOP (almost no transfusion-issue method is waived).
• Must not independently assign ABO/Rh for transfusion, interpret an AHG crossmatch, release a unit from the blood bank, or hang blood.

If a provider says “just give O-neg” in a clinic without a blood bank, follow emergency protocol by notifying the laboratory. Do not retrieve a unit and issue it.

Specimen: pink or lavender EDTA, special labeling, and fibrinogen

A type and screen or type and crossmatch typically requires EDTA whole blood so cells and plasma are available from one tube.

Special labeling is part of the test: two identifiers matching the order and armband, date and time, collector identification, and a blood-bank band number when the facility uses one. Hemolyzed (quality sense), unlabeled, or mismatched specimens are redraws, not relabels.

Fibrinogen (Factor I) is in AMT’s III.7 terminology list even though it is not a blood-group antigen. It is the plasma coagulation protein thrombin converts to fibrin (Chapter 11 overlap). Plasma contains fibrinogen; serum does not. Blood-bank EDTA plasma still contains fibrinogen. A tube that clotted—fibrinogen consumed, a clot sitting in “EDTA”—is not an acceptable typing specimen. Fibrin strands can cause false agglutination. That is why the outline parks fibrinogen next to blood, serum, plasma, and agglutination: it is the protein that distinguishes plasma from serum and that ruins a clotted EDTA sample.

Scenario

Two patients named Maria are in adjacent bays. A pink-top is labeled at the desk from a leftover printed label. That tube is not a type-and-screen specimen. Recollect at the correct bedside with the armband and required collector identification. Never “fix” the name on a blood-bank tube.

Exam traps

• Treating Rh as a fifth ABO type (A, B, AB, O, Rh).
• Believing CMLA certification authorizes AHG crossmatch or unit issue.
• Prelabeling a pink-top or using a leftover lavender CBC tube against policy.
• Calling fibrinogen an ABO antigen or assuming it must be removed before typing.
• Calling AB the universal red-cell donor (AB is the plasma-side teaching; O is the packed-red-cell teaching).

Name the type, name the tube, label at the bedside, and leave AHG interpretation and unit release to the blood bank.