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Seroconversion

The point at which antibody levels in a patient's blood reach the threshold for detection.

Timing of seroconversion in infections

It typically coincides with the onset of signs and symptoms of disease.

Early detection limitations in agglutination tests

Patients generally do not produce detectable levels of antibodies during the first week or longer of an infection.

Seroconversion duration in HIV infection

It generally takes about 3 weeks, although it can take much longer in some cases.

Purpose of serial two-fold dilutions in agglutination assays

They allow the titer of agglutinating antibody in a patient's serum to be determined.

Changes in antibody titers during infection

Antibody levels change over time during both primary and secondary immune responses.

Comparison of antibody titers for diagnosis

Comparing acute-phase and convalescent-phase titers can help determine whether an infection is current or occurred in the past.

Implications of changes in antibody titer

They can indicate how well the patient's immune system is responding to the pathogen.

Hemagglutination

The agglutination of red blood cells.

Direct Coombs' test (DAT)

A test that detects nonagglutinating antibodies or complement attached to red blood cells in vivo.

Another name for the direct Coombs' test

The direct antiglobulin test (DAT).

Why perform direct Coombs' test on newborns with jaundice

It can determine whether the newborn's RBCs were bound by maternal antibodies during pregnancy.

Maternal antibodies and newborn jaundice

Maternal antibodies can bind to the newborn's RBCs, activate complement, cause RBC lysis, and increase bilirubin levels.

Conditions producing positive direct Coombs' test

Hemolytic transfusion reactions, autoimmune hemolytic anemia, infectious mononucleosis, syphilis, and Mycoplasma pneumonia infections.

Type of antibody most commonly bound to RBCs in direct Coombs' test

IgG.

Why IgG bound to RBCs often doesn't cause visible agglutination

The orientation of IgG's antigen-binding sites and the relatively large size of RBCs make direct cross-linking unlikely.

Coombs' reagent

An antiserum containing antihuman IgG antibodies, sometimes combined with anti-complement antibodies.

How Coombs' reagent causes visible agglutination

It links IgG antibodies attached to neighboring RBCs, causing the cells to agglutinate.

Indirect Coombs' test (IAT)

A test that screens a person's serum for unbound antibodies against RBC antigens other than A and B.

Uses of indirect Coombs' test

It can screen pregnant people for antibodies that could cause hemolytic disease of the newborn and can be performed before blood transfusions.

Hemagglutination caused by some viruses

Viral surface proteins can bind to RBCs and cross-link the cells, causing visible agglutination.

Viral protein responsible for influenza hemagglutination

Hemagglutinin (H).

Viruses detectable by direct hemagglutination

Mumps and rubella viruses, in addition to influenza virus.

Direct hemagglutination assay (HA) purpose

It can detect viruses that directly cause visible agglutination of RBCs.

Measuring viral titer with direct hemagglutination assay

A virus-containing sample is serially diluted, and the highest dilution that still produces visible agglutination is the viral titer.

Well types used in viral hemagglutination assays

V-shaped or round-bottomed wells in a microtiter plate.

Diffuse mat of RBCs in hemagglutination assay

Viral agglutination is occurring, indicating the presence of agglutinating virus.

Dense RBC pellet at the bottom of a well in hemagglutination assay

No viral agglutination occurred; the RBCs sedimented normally.

Viral hemagglutination inhibition assay (HIA) purpose

A test that detects antiviral antibodies by determining whether they prevent a virus from agglutinating RBCs.

Determining antibody titer in HIA

It is the highest dilution of patient serum that still blocks viral agglutination.