Antibodi memiliki dua bagian utama: fragment pengikatan antigen (Fab) dan region kristalisasi (FC).
Bagian Fab berfungsi untuk mengenali dan mengikat antigen, sedangkan bagian FC mengikat reseptor sel imun yang berbeda untuk memicu fungsi efektor yang beragam.
Fungsi Utama Antibodi
Opsonisasi
Opsonisasi adalah proses di mana antibodi menempel pada patogen, menandainya untuk fagositosis oleh sel fagosit.
Netralisasi
Antibodi netralisasi dapat mengikat permukaan patogen seperti virus, mencegah mereka masuk ke dalam sel target dengan cara menghalangi reseptor spesifik.
Aktivasi Komplemen
Sistem komplemen terdiri dari protein plasma yang bekerja sama melawan infeksi. Protein komplemen mendeteksi antibodi yang terikat pada permukaan patogen, mengaktifkan protein lainnya dan membentuk kompleks serangan membran.
Pembentukan Kompleks Imun
Pengikatan beberapa antigen dan antibodi bersama-sama membentuk kompleks imun, yang membatasi kemampuan difusi antigen sehingga memudahkan fagositosis.
Sitotoksisitas Bergantung Antibodi
Antibodi dapat mengikat dan menghancurkan sel target seperti sel terinfeksi virus atau sel tumor. Sel pembunuh alami mengenali bagian FC dari antibodi dan melepaskan granula sitotoksik untuk memicu apoptosis pada sel target.
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Functions of Antibody
An antibody has two main parts which are involved in its functions. They are: fragment antigen-binding or Fab fragment and fragment crystallizable region that is Fc region. The Fab fragment is involved in recognizing and binding antigens. The Fc region binds to different immune cell receptors and evoke different effector functions that results in the destruction and removal of the antigens.
Different functions of an antibody are:
Opsonisation
Antibodies (mainly IgG1 and IgG3) can act as opsonins by binding to the pathogen through their paratopes, which allows better recognition by phagocytes. This antibody-antigen complex then binds to phagocytes through their Fc receptors and phagocytosis is initiated to eliminate the pathogen.
Neutralisation
Antibodies can prevent pathogens from accessing cells by blocking different parts of the bacterial or viral cell surface. Consequently, this neutralises certain viruses and bacterial toxins. IgG and IgA antibodies have the greatest effect.
Complement Activation
Complement system contains a collection of plasma proteins that work together to fight infection. In a classical complement pathway, a complement protein detects antibodies bound to the surface of a pathogen. This further activates other complement proteins resulting in the formation of the membrane attack complex – a cylindrical structure that is inserted into the cell wall of the pathogen, thus puncturing its membrane. Through this pore, fluids and molecules flow in and out of the cell. The cell swells and bursts.
Immune Complexes
The binding of multiple antigens and antibodies together can form immune complexes. Complex formation limits the antigens’ diffusing ability, making it easier for phagocytes to find and ingest pathogens through phagocytosis and simultaneous destruction of several pathogens.
Antibody-Dependent Cell-Mediated Cytotoxicity
Antibodies bind and opsonise target cells such as virus-infected host cells or tumor cells. Effector cells such as Natural killer cells recognize the Fc portion of the antibody and release cytotoxic granules (perforin and granzymes) into the target cell which triggers apoptosis.
References:
https://www.sinobiological.com/resource/antibody-technical/antibody-structure-function
https://byjus.com/biology/antibodies-role-of-antibodies/
https://teachmephysiology.com/immune-system/adaptive-immune-system/antibodies/
Kelly, J., 1992. Immunology: by Janis Kuby, WH Freeman.
https://en.wikipedia.org/wiki/Complement_system#/media/File:Complement_pathway.svg
https://www.creative-diagnostics.com/complement-system.htm
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