What is the difference between a primary antibody and a secondary antibody?

Question

Accepted Answer

Basis of differentiation
Primary antibody
Secondary antibody

Definition 
Is an immunoglobulin that binds directly to a specific protein or other target biomolecule 
Is designed to bind to the primary antibody, which is directly bound to the target antigen

Use in immunoassays
Is necessary to bind to the target antigen
Is not always necessary

Interaction with antigen
Interacts directly with the antigen
Interacts indirectly with the antigen by binding with the primary antibody

Source
Host species of the primary antibody must be different from the species of the sample
Host species of the secondary antibody must be different from the host species of the primary antibody

Function
Used as biomarkers for detecting diseases such as Alzheimer’s disease, cancer, Parkinson’s disease and diabetes. 
Used in immunolabeling

Advantages

- No risk of secondary antibody cross-reactivity
- More streamlined workflow & faster results
- Improved experimental reproducibility

- Increased sensitivity
- Multiplexing or multi-labeling capability
- More versatile 
- Extended access to a wider range of probes
- More flexibility in assay design

Basis of differentiation	Primary antibody	Secondary antibody
Definition	Is an immunoglobulin that binds directly to a specific protein or other target biomolecule	Is designed to bind to the primary antibody, which is directly bound to the target antigen
Use in immunoassays	Is necessary to bind to the target antigen	Is not always necessary
Interaction with antigen	Interacts directly with the antigen	Interacts indirectly with the antigen by binding with the primary antibody
Source	Host species of the primary antibody must be different from the species of the sample	Host species of the secondary antibody must be different from the host species of the primary antibody
Function	Used as biomarkers for detecting diseases such as Alzheimer’s disease, cancer, Parkinson’s disease and diabetes.	Used in immunolabeling
Advantages	No risk of secondary antibody cross-reactivity More streamlined workflow & faster results Improved experimental reproducibility	Increased sensitivity Multiplexing or multi-labeling capability More versatile Extended access to a wider range of probes More flexibility in assay design

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