Antigen

Antigens may be proteins, polysaccharides, lipids, nucleic acids or small chemicals, and come from microbes (microbial antigens), the environment (allergens), other individuals (alloantigens such as ABO and HLA), tumours (tumour antigens) or the body itself (autoantigens). Each antigen usually carries many epitopes (antigenic determinants). B-cell epitopes are recognised on the native molecule and may be conformational (formed by amino acids brought together by folding) or linear; antibodies can recognise any chemical class. αβ T-cell epitopes are short linear peptides displayed on MHC molecules: ≈ 8–10 amino acids on MHC class I and ≈ 13–25 on class II.

Immunogenicity — the ability to induce a response — depends on foreignness, size (large molecules > 10 kDa are more immunogenic), chemical complexity, the ability to be processed and presented (proteins are the best immunogens), dose, route, the presence of adjuvants or danger signals, and the host's genes (especially HLA type). Haptens are small molecules (e.g. penicillin, nickel ions, urushiol from poison ivy) that are not immunogenic alone but become so after binding a carrier protein — the basis of many drug allergies and contact dermatitis. Protein antigens are T-dependent; polymeric antigens such as capsular polysaccharides can activate B cells without T-cell help (T-independent), giving mainly IgM and little memory.

Superantigens are microbial toxins (staphylococcal toxic shock syndrome toxin-1 and enterotoxins, streptococcal pyrogenic exotoxins) that bypass normal recognition by clamping MHC class II to the side of the TCR Vβ region outside the peptide groove, activating up to ≈ 20% of all T cells at once instead of the usual tiny fraction — producing a massive cytokine release.