An introduction to immunology and immunopathology

Table 2  Major functions of human Ig antibodies [

5

]

Ig antibody Function

IgM

• First immunoglobulin (Ig) expressed during B cell 

development (primary response; early antibody)

• Opsonizing (coating) antigen for destruction

• Complement fixation

IgG

• Main Ig during secondary immune response

• Only antibody capable of crossing the placental barrier

• Neutralization of toxins and viruses

• Opsonizing (coating) antigen for destruction

• Complement fixation

IgD

• Function unclear; appears to be involved in homeostasis

IgA

• Mucosal response; protects mucosal surfaces from toxins, 

viruses and bacteria through either direct neutralization 

or prevention of binding to mucosal surface

IgE

• Associated with hypersensitivity and allergic reactions

• Plays a role in immune response to parasites

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Marshall et al. Allergy Asthma Clin Immunol 2018, 14(Suppl 2):49

bacteria, and cancer cells displaying tumour 

antigens;

  

• The activation of macrophages and NK cells, 

enabling them to destroy intracellular pathogens; 

and

  

• The stimulation of cytokine (such as IFNγ) 

production that further mediates the effective 

immune response.

Cell-mediated immunity is directed primarily at 

microbes that survive in phagocytes as well as those 

that infect non-phagocytic cells. This type of immunity 

is most effective in eliminating virus-infected cells 

and cancer cells, but can also participate in defending 

against fungi, protozoa, cancers, and intracellular 

bacteria. Cell-mediated immunity also plays a major 

role in transplant rejection.