Antigens medical comic

This comic explains what makes a molecule an antigen and what makes it able to start an immune response. It covers haptens and carriers, how B cells and T cells see epitopes differently, heterophile antigens used in diagnosis, T-dependent and T-independent antigens, superantigens, the factors behind immunogenicity and how adjuvants boost vaccines.

For MBBS, NEET-PG, FMGE, NEXT and USMLE revision.

Antigens: what the comic covers

High-yield points

• All immunogens are antigens, but not all antigens are immunogenic: antigenicity is the ability to bind antibody, immunogenicity the ability to trigger a response.
• A hapten is a small molecule that can bind antibody but becomes immunogenic only when joined to a carrier protein.
• B cells recognise conformational (3D) and linear epitopes; T cells recognise only linear peptides presented on MHC.
• Heterophile antigens shared across species underlie tests such as Weil–Felix (Proteus antigens for rickettsial antibodies) and Paul–Bunnell (sheep red cells for infectious mononucleosis).
• T-dependent (protein) antigens give class switching and memory; T-independent antigens such as polysaccharides and LPS give a fast IgM response with no memory.
• Superantigens bind MHC class II and the T-cell receptor V-beta region outside the groove, activating huge numbers of T cells — e.g. staphylococcal TSST-1 and enterotoxins, streptococcal pyrogenic exotoxins.
• Immunogenicity rises with foreignness, size, chemical complexity and the route and dose given.
• Adjuvants such as alum boost vaccine responses by slowing antigen release and drawing in phagocytes.

Mnemonics from the comic

• Epitopes: T cells read the Text (linear); B cells read the Body (conformational).

Quick questions

What is the difference between an antigen and an immunogen?

An antigen can bind an antibody or T-cell receptor. An immunogen can also start an immune response. Haptens are antigens that are not immunogenic until linked to a carrier.

How do superantigens work?

They bind MHC class II on antigen-presenting cells and the V-beta region of T-cell receptors outside the normal peptide groove, activating many T cells at once and causing a cytokine storm, as in toxic shock syndrome.

Why do polysaccharide vaccines work poorly in young children?

Polysaccharides are T-independent antigens: they give a short-lived IgM response without memory, and children under two respond poorly. Conjugating them to a protein makes them T-dependent.

Is this comic free to read?

The first pages are free to preview. Members read the full comic, and you can sign up free to read one comic in full.

Read next

• Antibodies medical comic
• Antigen–Antibody Reactions medical comic
• Immune Response medical comic
• Innate and Acquired Immunity medical comic

See membership plans to unlock every comic · Sign up free and read a comic