There are five major isotypes of immunoglobulin produced in humans. Immunoglobulins are generally named for the heavy chain type included in their structure, for example, IgM is based on µ (greek letter 'mu' used to denote 'micro') heavy chains, whereas IgG contains γ (greek letter 'gamma') chains.

IgG

This is the most abundant type of antibody found in humans. It accounts for ~80% of all circulating immunoglobulin molecules. It is produced primarily by memory B cells and plasma cells and circulates freely in the blood plasma and lymphatic fluid. It serves to bind to invading pathogens and other foreign antigens, triggering a signaling cascade that leads to complement activation and the recruitment of other immune cells.

Due to its production by memory cells, IgG can serve as a good marker of whether a patient has previously encountered or received a vaccine for a particular pathogen.

IgM

IgM is the first antibody to be produced by B cells when they encounter a pathogen for the first time. As such it serves as a good diagnostic marker for whether a patient is currently infected with a particular pathogen. Patients presenting with high IgM but no IgG for a particular disease are often retested a few days later to confirm infection.

Approximately 10% of the circulating antibody is of the IgM isotype. The major role of IgM is to activate the complement system and trigger an immune response. IgM can form a pentameric (5 molecules bound together) form that has ten overall binding sites for antigen. This huge molecule is most commonly found in the blood. IgM's general location and structure accounts for the molecule's critical role in activating immune responses to infections in the bloodstream.

The initiating molecule of the complement cascade, C1q, requires at least 2 molecules of IgG to be bound to a pathogen before it too can bind on top. IgM's pentameric form means that only a single IgM molecule needs to bind to a pathogen before C1q has enough binding sites to bind on top, meaning IgM can trigger the complement signaling cascade with great efficiency.

IgD

This is the isoform that is most abundantly expressed on the surface of B cell IgM is also expressed as a surface immunoglobulin.

IgA

A dimeric form the antibody molecule that also contains a secretory component. This antibody is produced and secreted by the mucosal lymphoid tissues and so is found mainly in secretions such as mucus, saliva, and tears. IgA can bind and subsequently inactivate pathogens that find their way into the mucus of the nose or gut, helping them to be swept along and destroyed by digestion processes further along the way.

IgE

A special kind of heavyweight antibody that is involved in allergic and antiparasitic responses only. It serves to signal a response requirement from mast cells and basophils. Mast cells are strongly triggered to degranulate on binding to this antibody, leading to bodily responses such as coughing, sneezing and other symptoms associated with allergic responses.