An antibody catalog entry packs three facts into a few words. "Rabbit anti-human PSD-95 IgG" tells you the host it was raised in, the target it binds, and the immunoglobulin class it belongs to, and each of those changes how you can use it.
Host
The host is the animal that was immunized to make the antibody. It matters for two practical reasons. Your secondary antibody has to recognize the host's immunoglobulin, so a rabbit primary needs an anti-rabbit secondary. And the host should be a different species from your sample, otherwise the secondary will also bind the sample's own immunoglobulin and give you background. That's why rabbit and goat antibodies are convenient on mouse tissue, and why a mouse primary on mouse tissue needs extra blocking. There's more on this in choosing a host species.
Target
The target is the antigen, and the entry usually names the species it came from ("anti-human", "anti-rat"). Many proteins are conserved, so an antibody often cross-reacts with the same protein in other species. A good datasheet lists the species it has actually been tested against instead of leaving you to guess. Specificity, the ability to bind that target and nothing else, has to be demonstrated rather than assumed. Look for a Western blot with a single band at the right size, or staining that matches the known distribution of the protein. Our antibody pages carry that evidence for each product.
Two related terms come up constantly. Polyclonal antibodies are the mixed population from an immunized animal's serum and recognize several epitopes on the target. Monoclonal antibodies come from a single B-cell clone and recognize one. Polyclonals give more signal and tolerate denaturation better; monoclonals are more uniform from lot to lot.
Isotype
Mammals make five classes of antibody, and the class shapes what the molecule does in an assay.
IgG is the class almost every research antibody belongs to. It's stable and monomeric, it binds Protein A/G for purification, and it works in Western blot, ELISA, IHC and flow cytometry. Subclasses (IgG1, IgG2a and so on) matter mainly when you choose isotype controls and secondaries.
IgM is a pentamer with ten binding sites. Its size makes it a strong agglutinator and complement activator, which is useful in blood typing and cytotoxicity work, but it penetrates tissue poorly and doesn't bind Protein A/G.
IgA is the secretory antibody of mucosal surfaces. In research it shows up mostly as the analyte, for example when measuring mucosal vaccine responses. IgE mediates allergic responses and is the target of allergy assays rather than a tool. IgD is rare in serum and seldom used as a reagent.
When you buy a purified immunoglobulin rather than an antibody against a target, the isotype is the product. Our human IgG and human IgM are affinity purified for use as calibrators, controls and blocking proteins in immunoassays.
Choose the host to fit your sample and the other antibodies you'll use alongside it. Confirm the target with real validation data in your application. Default to IgG unless the assay needs IgM's avidity. And if you're unsure whether one of our antibodies will work in your system, ask us and we'll tell you what it has and hasn't been tested in.