Antibody- Mediated/ Humoral Immunity

What are antibodies and what is their role in immunity?

• Glycoprotein molecules which are specific their antigen

• Produced by B-lymphocytes

• Recognises proteins, carbohydrates, nucleic acids, glycolipids and small inorganic molecules

Mediates responses such as;

• Cell lysis/attach innate cell, complement

• Interact with phagocytic cells via constant region (Fc) of antibody

• Can have a neutralising effect - block viral entry into cell

What determines antibody isotype and structure?

The constant region (Fc) determines antibody (Ab) isotype and structure. Fc receptors on cells bind with different affinity for antibody subclasses.

What is the role of cytokines in antibodies?

The isotype secreted by plasma B cells are associated with the cytokines produced by T helper cells. This then determines the response activated.

What are the functions of antibodies?

Three key functions;

1. Neutralisation - Antibody binds to toxin, preventing the toxin from interacting with target protein. Can be ingested by macrophage. (IgG, IgA)

2. Opsonisation - Bacteria in extracellular space, antibodies bind to surface of bacteria. Fc region remains free to interact with Fc receptors on macrophages for ingestions.

3. Complement - Bacteria in plasma, antibodies coat bacteria and recruit complement components to form MAC through enzyme cascades and can be taken in by a macrophage.

These functions all depend on the variable region and its antigen-specific binding.

How do antibodies mediate NK cell killing?

Antibodies bind to the antigens on the surface of target cells. Fc receptors on the NK cell recognise the Fc of the antibodies and bind to them, this signals NK cell to kill the target cell via apoptosis.

Describe IgM

• Pentamer

• First class secreted in immune response and has a short half life

• Present in babies

• Produced in response to antigenic stimulation of B cells - adaptive

• Broad range of microbial components

• High avidity of polymeric IgM, can immobilise targets

Describe IgG

• Secreted later in primary response and main antibody body class in secondary response

• Long term protection, gives baby protection before immune system fully develops

• >75% of Igs in blood

• Mediates phagocytosis and opsonisation

• Mediates NK cells

• Mediates degranulation of neutrophils, eosinphils and mast cells

Describe IgA

• Monomeric or Dimeric

• (Dimeric) survives on mucosal surfaces

• Mainly in gut

• Can be deficient

• Important against intestinal pathogens (Rotavirus)

• In GI tract and protects against HIV

Describe IgD

• Antigen naïve B-cells express IgD (and IgM)

• Short half life

• Serum concentration varies, might be genetic or environmental

• Serum concentration increase in patients with respiratory pathogens

• Might be protective against mucosal pathogens

• IgD response may increase in response to IgA deficiency

Describe IgE

• Evolved in response to worm infections but in 1st world countries is now associated with allergic reactions

• Mediated via degranulation of granulocytes

• Activation occurs through receptors on mast cells, basophils and multivalent antigens. Triggers degranulation. In mast cells, triggers release of enzymes that inactivate venom.

• Induces T helper 2 immune response with inflammation