Thirty minutes in a 56 °C water bath changes serum in one useful way: it destroys the complement. Heat-inactivated serum is how you get the growth factors, hormones and carrier proteins of serum without the lytic activity, and it's the negative control that gives a complement assay its meaning.
What the heat does
Complement proteins are among the most heat-labile things in serum. At 56 °C, C1 and C2 lose activity within minutes and Factor B soon after. By thirty minutes the classical and alternative pathways are both gone. Most other serum proteins tolerate the treatment, although some heat-sensitive enzymes and growth factors are reduced, and heating longer or hotter causes aggregation and turbidity. That's why the protocol is so specific: 56 °C, thirty minutes, bottle mixed so it heats evenly, then cooled promptly.
When you need it
Cell culture is the classic case. Serum in culture medium supports growth, but active complement can lyse cells that carry bound antibody or activate complement on their own, and it interferes with assays that use complement on purpose. Heat inactivation used to be routine for all culture serum. Now it's used where the cell type or the experiment calls for it, such as hybridoma work, primary lymphocyte culture and complement-dependent cytotoxicity controls.
Complement assays need it as the control. Every serum bactericidal or cytotoxicity assay needs a condition that has everything the active serum has except complement. Killing seen with active complement and absent with heat-inactivated serum proves the effect is complement-mediated. We supply heat-inactivated human serum as that complement-free control alongside the active antibody-depleted complement source (see choosing the right antibody-depleted human serum).
Immunoassays and flow cytometry are the third case. Serum used as a blocking agent or diluent shouldn't carry active complement, which can deposit on cells and beads and add background. Heat-inactivated serum blocks without that side effect.
Doing it yourself or buying it done
Inactivating serum in the lab is easy to do and easy to do inconsistently. Bath temperature varies, bottles get pulled early or left in too long, and the serum is often refrozen afterwards, which degrades it further. Serum heat inactivated under controlled conditions has had the same treatment every lot and behaves the same way in your assay. If you do it in house: thaw the serum fully first, bring the bath to temperature before the bottle goes in, time from when the serum itself reaches 56 °C, cool it in a water bath, and aliquot before you freeze it.
We offer sterile heat-inactivated serum from goat, rabbit, pig and human, so what you receive is both sterile and complement-free. If you need another species, or a heat-inactivated version of a serum you already buy from us, ask; it's a routine custom request.