Sel Limfosit B dapat membentuk struktur protein khusus yang disebut antibodi. Protein khusus ini dimigrasikan ke bagian membran sel, kemudian berfungsi mengenali dan mengikat sel asing atau organisme asing yang ditemui, dan melumpuhkannya. Antibodi pada dasarnya adalah protein yang sangat spesifik yang terbentuk sebagai respons dari kehadiran antigen. 

Antibodi tersusun atas dua tipe rantai polipeptida yaitu rantai ringan (light chain) dan  rantai berat  (heavy chain). Struktur gabungan kedua rantai tersebut membentuk huruf Y. Di tengah-tengah ikatan rantai tersebut terdapat  daerah Hinge (Hinge Region) yang memungkinkan rantai-rantai polipeptida untuk bergerak. Setiap lengan dari antibody memiliki daerah pengikat antigen (antigen-binding site).

Umumnya antibodi terdiri atas sekelompok protein yang berada pada fraksi-fraksi globulin serum. Fraksi-fraksi globulin serum ini dinamakan imunoglobulin atau disingkat Ig. Immunoglobin (disingkat Ig) dibagi menjadi lima kelas, yaitu IgA, IgD, IgE, IgG, IgM. Perhatikan gambar berikut ini.

IgM merupakan antibodi pertama yang disekresikan sebagai respons kekebalan tubuh. Setelah mengikat antigen, IgM memicu aktifnya protein komplemen. IgM juga dapat mengikat antigen atau patogen menjadi gumpalan sehingga memudahkan fagositosis makrofag.

IgG mengaktifkan protein komplemen dan menetralkan banyak racun. Jumlah IgG paling banyak dan tahan lama. IgG merupakan satu-satunya antibodi yang dapat melewati plasenta dan menjaga janin dengan kekebalan tubuh ibunya. IgG juga disekresikan dalam kolostrum.

IgA mencegah masuknya virus atau bakteri melalui jaringan epitel mukosa sistem pencernaan, pernapasan, dan saluran reproduksi. IgA ditemukan juga pada air liur, air mata, dan kolostrum.

IgE memicu peradangan jika cacing parasit menyerang tubuh. IgE juga berperan dalam reaksi alergi.

IgD tidak mengaktifkan sistem komplemen dan tidak dapat melewati plasenta. IgD diduga berfungsi dalam diferensi sel limfosit B menjadi sel plasma dan sel B memori.

Sumber : Buku Biologi XI Pengarang Fictor Ferdinand P dan Moekti Ariebowo.

Demikian saya ucapkan terima kasih untuk kunjungannya, semoga bermanfaat bagi kita semua. Silahkan kasih komentar jika ada yang hal-hal yang ingin ditanyakan tentang Materi dan Soal Biologi SMP SMA Rembang. Atau contact saya di no HP. 085641467626. Atau via email di

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