serum

IPA/ˈsɪərəm/
KK[sˈɪrəm]IPA/ˈsɪrəm/

serum — noun

  • serumsingular
  • serumsplural

1. The clear, pale-yellow liquid part of blood that stays after the blood clots and

1.nounB2
Definition

The clear, pale-yellow liquid part of blood that stays after the blood clots and all the solid cells and clotting agents are taken out.

Example

The lab technician separated the serum from the blood sample for further testing.

passive: separated from blood for analysis

Doctors check the level of vitamin D in a patient's blood serum to diagnose deficiencies.

check level of [substance] in blood serum

Common collocations
  • blood serum
  • serum sample
  • serum level
  • separate the serum
  • in the serum
Synonyms
  • plasma

    plasma is the liquid part of blood before clotting factors are removed; serum is plasma without those factors.

Grammar Patterns

serum of [blood/sample]

in/from the serum

Usage Note

Often used in expressions that describe medical tests measuring the amount of a substance in the blood, such as 'serum iron' or 'serum cholesterol'.

Common Mistakes

The serum in the blood is red.
The serum is a clear, pale-yellow liquid.
whole blood is red because of red blood cells; serum is the liquid left after those cells are removed.

2. A medical preparation taken from an immunised animal or person whose body has bu

2.nounB2
Definition

A medical preparation taken from an immunised animal or person whose body has built protection against a disease, given to someone else to help them fight that disease or a poison.

Example

After the snake bit his leg, the hospital gave Brian a dose of antivenom serum.

serum against poison (antivenom)

Doctors injected the patient with a serum to stop the infection from spreading further.

passive: injected with a serum

Common collocations
  • antivenom serum
  • inject a serum
  • receive a serum
  • serum against rabies
Synonyms
  • antiserum

    antiserum is the exact technical term; 'serum' is the shortened everyday form.

  • antidote

    antidote neutralises a poison chemically; serum works through antibodies from an immune system.

Grammar Patterns

serum against [disease/poison]

inject/give [someone] a serum

Usage Note

Differs from a vaccine: a serum contains ready-made antibodies from another person or animal, while a vaccine stimulates your own body to make antibodies.

Common Mistakes

The doctor gave me a vaccine against the snake venom.
The doctor gave me a serum against the snake venom.
vaccines prevent diseases you might catch; serums can treat poison or infection you already have.

3. A thick liquid cosmetic product that packs a high amount of active ingredients a

3.nounB1
Definition

A thick liquid cosmetic product that packs a high amount of active ingredients and aims to boost the quality or appearance of the skin or hair.

Example

Mizuki applies a vitamin C serum to her face every morning before putting on cream.

apply [type] serum to [body part]

This hair serum repairs split ends and adds shine without making hair greasy.

hair serum — specific product category

Common collocations
  • vitamin C serum
  • hydrating serum
  • hair serum
  • face serum
  • apply serum
  • dropper of serum
Synonyms
  • essence

    in skincare, 'essence' is typically lighter and more watery than a serum; serums are more concentrated.

  • concentrate

    a marketing term that overlaps with serum but can also refer to thicker treatments.

Grammar Patterns

[adjective] serum

apply serum to [area]

serum for [hair/skin]

Usage Note

Beauty serums are more concentrated than regular moisturisers or lotions. They are usually sold in small bottles with a dropper and applied in small amounts.

Common Mistakes

I put a thick layer of serum all over my face.
I put a few drops of serum on my face and gently spread it.
serums are concentrated; using too much can waste the product and irritate the skin.

4. A thin, watery liquid produced naturally in certain body tissues, such as the cl

4.nounC1
Definition

A thin, watery liquid produced naturally in certain body tissues, such as the clear fluid that fills a skin blister or lubricates the space around internal organs.

Example

The blister on Esme's heel was filled with clear serum that protected the new skin.

serum filling a blister

Serum collected around the surgical wound during the healing process.

Common collocations
  • serum in the blister
  • serum around the wound
  • build up of serum
  • clear serum
Synonyms
  • serous fluid

    the more technical medical term; 'serum' in this sense is a shorter form used in clinical settings.

  • lymph

    lymph is a similar fluid but circulates through the lymphatic system; serum in tissues is not part of that system.

Grammar Patterns

serum in [body part/tissue]

Usage Note

In medical contexts this overlaps with 'serous fluid'. It is thinner than pus and does not contain the same infection-fighting white blood cells.

serum — adjective

Word family

  • serous
    adjectiveC1
  • serology
    nounC1
  • serological
    adjectiveC1
  • antiserum
    nounC1

Synonym discussion

Sense: noun/1

All three terms refer to bodily fluids, but each has a distinct meaning. 'Whole blood' is blood with all its components — red cells, white cells, platelets, and plasma — and is the everyday term for the red liquid in veins. 'Plasma' is the liquid part of blood before clotting occurs; it contains clotting factors such as fibrinogen. 'Serum' (noun sense 1) is what remains after blood has clotted and those clotting factors have been removed, making it slightly different in composition from plasma. 'Lymph' is a different fluid that circulates through the lymphatic system, not the blood vessels. In a medical test, a doctor might say 'we checked your serum iron' (not plasma iron, because the test uses clotted blood). If you donate blood, the bag contains whole blood; but if the lab needs serum, they let the blood clot first and then spin it in a centrifuge to separate the clear liquid.

plasma ↗whole bloodlymph ↗

Common collocations

Nouns

  • blood serum
  • serum level
  • serum sample
  • serum concentration
  • serum test
  • serum protein
  • serum cholesterol

Phrases

  • separate the serum
  • inject a serum
  • measure serum levels

Prepositions

  • in serum
  • of serum
  • from the serum

Etymology

LatinEnglish

The word comes from Latin 'serum', meaning 'whey' — the watery part of milk that separates from the curd during cheesemaking. Early medical writers in the 1600s borrowed the term because the clear liquid left after blood clots looks similar to whey separating from milk. Over time the word developed specialised uses in immunology and, more recently, in the cosmetics industry.