(h) the structure and general functions of antibodies

The idea

Antibodies are globular glycoproteins of the immunoglobulin family, secreted by plasma cells (differentiated B lymphocytes) in response to a specific antigen. Each antibody is a Y-shaped molecule of four polypeptide chains: two identical heavy chains and two identical light chains, held by disulfide bridges. The variable regions at the tips of the two arms form the antigen-binding sites, giving each antibody two identical binding sites and a shape complementary to one specific antigen. The constant regions are the same within a class and allow recognition by phagocytes and complement. General functions: binding to antigen to neutralise pathogens or toxins, marking them for destruction, and forming antigen–antibody complexes that can be destroyed by phagocytosis. Binding is specific and reversible.

What earns the marks

• Antibodies are proteins (globular glycoproteins/immunoglobulins) secreted by plasma cells derived from B lymphocytes.
• Each antibody has four polypeptide chains: two heavy and two light chains, joined by disulfide bridges.
• The Y-shaped molecule has two variable regions forming two identical antigen-binding sites.
• The binding site shape is complementary to a specific antigen, so binding is specific.
• Constant regions are the same within a class and allow recognition by phagocytes/complement.
• General functions include neutralising pathogens or toxins, marking antigens for destruction and forming complexes removed by phagocytosis.

Common mistakes and how to put them right

• Saying antibodies are produced by phagocytes or T cells.

  Put it right: Correct that they are secreted by plasma cells (from B lymphocytes).

• Describing the binding site as complementary to the antibody rather than to the antigen.

  Put it right: Correct that the variable region is complementary to a specific antigen.

• Claiming antibodies directly kill pathogens.

  Put it right: Correct that they bind, neutralise and mark antigens for destruction by phagocytosis or complement.

Exam tips

• Learn the four-chain Y structure with two heavy and two light chains and label the variable and constant regions.
• Always link structure to function: variable region gives specificity, constant region gives recognition by phagocytes.
• Use the term 'complementary' when describing antigen–antibody binding rather than 'matching' loosely.