Antibody Overview

Antibodies bind to viruses and microbial toxins, thereby inactivating them. The binding of antibodies to invading pathogens also recruits various white blood cells (WBCs), and a system of blood proteins, collectively called the complement.

Antibodies, collectively called immunoglobulins (Ig), are produced in billions of forms, each with different amino acids and a different antigen binding site. Igs are the most abundant protein components in the blood, constituting ~20% of total protein in plasma by weight.

Mammals have five classes of antibodies which mediate a characteristic biological response following antigen binding.

Cell Surface Receptors and Secreted Molecules

As predicted by clonal selection theory all antibodies made by an individual B cell have the same antigen binding site.

The first antibodies made by newly formed B cells are not secreted; they're inserted into the plasma membrane where they serve as receptors for antigens. Each B cell has about ${10}^5$ such receptors in its plasma membrane. Receptors are associated with a complex of transmembrane proteins that activate intracellular signalling pathways when they bind to antigens.

Each B cell produces a single species of antibody with a unique antigen binding site. When a naïve or memory B cell is activated by an antigen (with the aid of T helper cells), it proliferates and differentiates into an antibody secreting effector cell.

Effector cells secrete a large amounts of soluble antibodies with the same unique antigen binding site as the cell surface antibody that serves as antigen receptors. Effector B cells can begin secreting when they're still small lymphocytes but the end stage of their maturation pathway is large plasma cells which secrete antibodies at a rate of ~2000 molecules per second.

Antibody Structures

The simplest antibodies are Y shaped molecules with two identical binding sites. Because of their typical structure they're widely described as bivalent.

As long as an antigen has three or more antigenic determinants (i.e. binding sites), bivalent antibody molecules can cross-link into a large lattice. These lattices can readily precipitate out of a solution and are rapidly phagocytosed and degraded by macrophages. Binding and cross linking efficiency is greatly increased by a flexible hinge region in most antibodies allowing for the distance between binding sites to vary.

Antibodies not only bind, but engage in a variety of activities mediated by one of several types of tail regions.

Functional properties can include:

• Ability to activate complement system
• Ability to bind to phagocytic cells
• Ability to cross placenta from parent to fetus

Source

Source: Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. B Cells and Antibodies. Available from: https://www.ncbi.nlm.nih.gov/books/NBK26884/

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