Primary antibodies are immunoglobulins that bind directly to specific antigens with high specificity and affinity. They can be developed either as monoclonal antibodies or polyclonal antibodies using mice, goats, rabbits, rats, and other animals as hosts. Monoclonal antibodies are derived from a single B cell and can only recognize and bind to one unique epitope (binding site) on the antigen. Polyclonal antibodies are produced from a collection of B cells and can bind to multiple epitopes on the same antigen. 

Primary antibodies are used in immunofluorescence assays for the purposes of detecting, sorting, purifying, and measuring the antigens in a sample. Because they can also be raised to recognize post-translational modifications such as methylation, phosphorylation, and acetylation, they can also be used to study components of living cells at the molecular level and to identify potential disease-causing proteins.