USMLE® Step 1 style questions USMLE
Content Reviewers:Rishi Desai, MD, MPH
Contributors:Tanner Marshall, MS, Jake Ryan, Elizabeth Nixon-Shapiro, MSMI, CMI, Kara Lukasiewicz, PhD, MScBMC
When you get an infection, you develop adaptive immunity.
In other words, you generate memory T and B cells, so that if you encounter the same antigen again, they can quickly replicate and respond.
That’s the key - getting long term active protection to a harmful microbe, from something that’s not harmful.
The amount of people within a herd that need to be vaccinated to maintain herd immune status differs from pathogen to pathogen.
When too few people in a herd are vaccinated, there are more people in the population that are able to get the illness and spread it.
This is different from passive immunity which is where a person gets antibodies that are made by another person or animal like a horse or mouse or by cells in a lab.
A common form of this is when antibodies are pooled from the community and is given intravenously - it’s called intravenous immunoglobulin or IV-Ig.
Passive immunity last for only as long as the antibodies last - usually weeks to months.
IgG antibodies in the blood cross the placenta initially protecting the baby to some pathogens that mom has already made antibodies to.
These IgG maternal antibodies will be degraded around six months of age.
IgA antibodies are plentiful in breast milk and are passed to the baby during nursing, these antibodies provide protection from pathogens that may be found at mucosal sites.
Once a baby weans off of breastmilk, these antibodies are no longer passed and the ones that have already entered the baby slowly degrade over the course of a few months. Fortunately, by that point the baby will begin to make some of their own antibodies.
When a patient receives a vaccine, CD4+ helper T cells are activated and produce cytokines like IFN gamma, TNF alpha, and IL-2 to promote growth of immune cells and class switching of activated B cells.
The exact antibody response depends on the route and type of vaccine.
Live attenuated and inactivated vaccines are whole cell vaccines, which means that the whole virus or bacteria is used to create the vaccine. Subunit vaccines - which includes polysaccharide vaccines, and Toxoid are considered fractionated vaccines because only one part of the pathogen is used to create the vaccine.
Live vaccines are attenuated, meaning that the pathogen has been weakened in the laboratory to make it less pathogenic, but still able to replicate in the vaccinated person so that it can stimulate an immune response.
Inactivated vaccines use a pathogen that has been killed using heat or chemical fixation with formalin.
Subunit vaccines that contain just the portions of pathogens that our bodies response to - like polysaccharides or proteins.
Often proteins from several different pathogens are conjugated or attached together to form conjugate subunit vaccines.