Editing Immune system (section)

===Humoral immune response===
{{further|Humoral immunity}}
[[File:2220 Four Chain Structure of a Generic Antibody-IgG2 Structures.jpg|thumb|upright=1.6 |alt=diagram showing the Y-shaped antibody. The variable region, including the antigen-binding site, is the top part of the two upper light chains. The remainder is the constant region. |An antibody is made up of two heavy chains and two light chains. The unique variable region allows an antibody to recognize its matching antigen.<ref name=NIAID>{{cite web|title=Understanding the Immune System: How it Works |publisher=[[National Institute of Allergy and Infectious Diseases]] (NIAID) |url=https://www.niaid.nih.gov/publications/immune/the_immune_system.pdf |access-date=1 January 2007 |url-status=dead |archive-url=https://web.archive.org/web/20070103005411/http://www.niaid.nih.gov/Publications/immune/the_immune_system.pdf |archive-date=3 January 2007 }}</ref>]]
A [[B cell]] identifies pathogens when antibodies on its surface bind to a specific foreign antigen.<ref name=Sproul>{{cite journal | vauthors = Sproul TW, Cheng PC, Dykstra ML, Pierce SK | s2cid = 6550357 | title = A role for MHC class II antigen processing in B cell development | journal = International Reviews of Immunology | volume = 19 | issue = 2–3 | pages = 139–55 | year = 2000 | pmid = 10763706 | doi = 10.3109/08830180009088502 }}</ref> This antigen/antibody complex is taken up by the B cell and processed by [[proteolysis]] into [[peptide]]s. The B cell then displays these antigenic peptides on its surface MHC class II molecules. This combination of MHC and antigen attracts a matching helper T cell, which releases [[lymphokine]]s and activates the B cell.<ref>{{cite journal | vauthors = Parker DC | title = T cell-dependent B cell activation | journal = Annual Review of Immunology | volume = 11 | pages = 331–60 | date = 1993 | pmid = 8476565 | doi = 10.1146/annurev.iy.11.040193.001555 }}</ref> As the activated B cell then begins to [[cell division|divide]], its offspring ([[plasma cells]]) [[secretion|secrete]] millions of copies of the antibody that recognizes this antigen. These antibodies circulate in [[blood plasma]] and [[lymphatic system|lymph]], bind to pathogens expressing the antigen and mark them for destruction by [[Complement system|complement activation]] or for uptake and destruction by [[phagocyte]]s. Antibodies can also neutralize challenges directly, by binding to bacterial toxins or by interfering with the receptors that viruses and bacteria use to infect cells.{{sfn| Murphy |Weaver |2016 |loc= Chapter 10: The Humoral Immune Response}}

Newborn infants have no prior exposure to microbes and are particularly vulnerable to infection. Several layers of passive protection are provided by the mother. During pregnancy, a particular type of antibody, called [[Immunoglobulin G|IgG]], is transported from mother to baby directly through the [[placenta]], so human babies have high levels of antibodies even at birth, with the same range of antigen specificities as their mother.<ref>{{cite journal | vauthors = Saji F, Samejima Y, Kamiura S, Koyama M | title = Dynamics of immunoglobulins at the feto-maternal interface | journal = Reviews of Reproduction | volume = 4 | issue = 2 | pages = 81–89 | date = May 1999 | pmid = 10357095 | doi = 10.1530/ror.0.0040081 | s2cid = 31099552 | url = http://pdfs.semanticscholar.org/ef69/261377c9d417b646679b850fc88a19128579.pdf | archive-url = https://web.archive.org/web/20210130212930/http://pdfs.semanticscholar.org/ef69/261377c9d417b646679b850fc88a19128579.pdf | url-status = dead | archive-date = 2021-01-30 }}</ref> Breast milk or [[colostrum]] also contains antibodies that are transferred to the gut of the infant and protect against bacterial infections until the newborn can synthesize its own antibodies.<ref>{{cite journal | vauthors = Van de Perre P | title = Transfer of antibody via mother's milk | journal = Vaccine | volume = 21 | issue = 24 | pages = 3374–76 | date = Jul 2003 | pmid = 12850343 | doi = 10.1016/S0264-410X(03)00336-0 }}</ref> This is [[passive immunization|passive immunity]] because the [[fetus]] does not actually make any memory cells or antibodies—it only borrows them. This passive immunity is usually short-term, lasting from a few days up to several months. In medicine, protective passive immunity can also be [[intravenous immunoglobulin|transferred artificially]] from one individual to another.<ref name= Keller>{{cite journal | vauthors = Keller MA, Stiehm ER | title = Passive immunity in prevention and treatment of infectious diseases | journal = Clinical Microbiology Reviews | volume = 13 | issue = 4 | pages = 602–14 | date = Oct 2000 | pmid = 11023960 | pmc = 88952 | doi = 10.1128/CMR.13.4.602-614.2000 }}</ref>