Advances in Bioscience and Biotechnology, 2013, 4, 21-33 ABB doi:10.4236/abb.2013.44A004 Published Online April 2013 (http://www.scirp.org/journal/abb/ ) Fc receptors: Cell activators of antibody functions Carlos Rosales 1* , Eileen Uribe-Querol 2 1 Immunology Department, Instituto de Investigaciones Biomédicas, Universidad Nacional Autónoma de México, Mexico City, Mexico 2 División de Estudios de Posgrado e Investigación, Facultad de Odontología, Universidad Nacional Autónoma de México, Mexico City, Mexico Email: * carosal@unam.mx Received January 18 th , 2013; revised March 12 th , 2013; accepted April 16 th , 2013 Copyright © 2013 Carlos Rosales, Eileen Uribe-Querol. This is an open access article distributed under the Creative Commons At- tribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is prop- erly cited. ABSTRACT At the onset of an infection early defense systems, such as complement, get into action. Specialized leu- kocytes (white blood cells) of the innate immune sys- tem, including monocytes, macrophages, and neutro- phils also participate as a first line of defense against infections. These early responses are rapid but not very specific and are usually not enough to clear com- pletely many infections. The adaptive immune system is also needed to finish the job against many micro- organisms. Antibody molecules, produced during the adaptive immune response, are crucial for preventing recurrent infections. Although, IgG antibodies are essential for controlling infections, these molecules do not directly damage the microorganisms they recog- nize. Today, it is established that leukocytes of the innate immune system are responsible for the protec- tive effects of these antibodies. IgG molecules bind to their cognate antigens and are in turn recognized by specific receptors (Fcγ receptors) on the membrane of leukocytes. Crosslinking these receptors on the sur- face of leukocytes leads to activation of several effec- tor cell functions. These effector functions are geared toward the destruction of microbial pathogens and the induction of an inflammatory state that is benefi- cial during infections. However, in autoimmune dis- eases, antibodies can direct these effector functions against normal tissues and cause severe tissue damage. In recent years, several factors that can modulate the IgG-FcγR interaction have been elucidated. In this review, we describe the main types of Fcγ receptors, and our current view of how antibody variants inter- act with these receptors to initiate different cell re- sponses. In addition, new findings on the signaling role of individual Fcγ receptors are also discussed. Keywords: Immunoglobulin; Antibody; Immunoreceptor; Neutrophil; Macrophage 1. INTRODUCTION At the onset of an infection by different types of micro- organisms, including viruses, bacteria, fungi, and proto- zoa, early defense systems, such as constitutive expres- sion of antimicrobial peptides, and activation of com- plement get into action. These systems are rapid but not particularly specific. Specialized leukocytes (white blood cells) of the innate immune system, including monocytes, macrophages, and neutrophils also participate as a first line of defense against infections. These leukocytes can bind some microbial molecules, termed danger- and pathogen-associated molecular patterns (DAMPs and PAMPs, respectively) via numerous receptors such as the Toll-like receptor family [1,2]. In this way, leukocytes recognize microorganisms directly and prevent a massive infection [3]. These early responses however, are usually not enough to clear completely many infections. The adaptive immune system is also needed to finish the job against many microorganisms. Antibody molecules, pro- duced during the adaptive immune response, are crucial for preventing recurrent infections [4]. At the beginning of the adaptive response, antibodies belong to the IgM class. These antibodies present low affinity for microbial antigens, but they can easily activate the classical com- plement pathway. Complement deposited on microor- ganisms can induce phagocytosis via complement recep- tors [5,6], or it can induce bacterial lysis via the forma- tion of the membrane attack complex [7]. At later times of the adaptive response, antibodies belong mainly to the IgG class. These antibodies are of higher affinity and of much greater specificity for their particular antigen. Thus, IgG antibodies are key for controlling many microorgan- isms, as demonstrated by immunodeficiency disorders, * Corresponding author. Published Online April 2013 in SciRes. http://www.scirp.org/journal/abb