In birds, B-lymphocyte maturation occurs in the bursa of Fabricius. In classical mammalian immunology, gut-associated lymphoid tissues such as Peyer's patches were designated as the bursa-equivalent organ (and selected in NTA official key), though in adult mammals primary B cell genesis occurs in the bone marrow.

The joining (J) chain is a $15\text{ kDa}$ polypeptide required for the polymerization and secretory transport of polymeric immunoglobulins: dimeric IgA (B) and pentameric IgM (C). Monomeric antibodies (IgG, IgD, IgE) lack a J chain.

Classification of immunoglobulin isotypes based on heavy chains:

• IgG possesses Gamma ($\gamma$) heavy chains (matching III).
• IgM possesses Mu ($\mu$) heavy chains (matching IV).
• IgA possesses Alpha ($\alpha$) heavy chains (matching I).
• IgE possesses Epsilon ($\epsilon$) heavy chains (matching II).

Pioneering contributions to humoral immunity:

• Emil von Behring won the first Nobel Prize in 1901 for serum therapy against diphtheria.
• Paul Ehrlich was awarded the 1908 Nobel Prize in Physiology or Medicine (shared with Ilya Metchnikov) for his work on immunity, specifically his revolutionary "Side-Chain Theory" (Seitenkettentheorie) of antibody formation, which correctly postulated that cells display pre-existing side-chain receptors that bind toxins and are subsequently shed as free antibodies into circulation.
• Louis Pasteur pioneered anthrax and rabies vaccines in the 19th century.
• Karl Landsteiner won the Nobel Prize in 1930 for the discovery of human ABO blood groups.