B cells and antibodies

Each antibody's arm tips fit one antigen's shape, the way a key fits one lock. Binding peptides held in MHC is how T cell receptors work, not antibodies.

• Any foreign molecule it meets:
• Correct: One particular antigen shape:
• Only peptides held in MHC proteins:

Helper T cells carry CD4, recognize antigen in MHC class II, and coordinate other cells, including B cells, with cytokines and surface contact.

• Cytotoxic T cell:
• Regulatory T cell:
• Correct: Helper T cell:

Activated complement proteins coat microbes (opsonization), attract phagocytes and build the membrane attack complex, which makes holes in the microbe's membrane.

• Correct: It coats the bacterium for phagocytes and can punch holes in its membrane:
• It makes the bacterium divide faster:
• It presents the bacterium's peptides to T cells:

The first exposure left memory B cells that outnumber the original naïve cells, respond with less stimulation, and already make switched, better-fitting antibody. When the same antigen returns, they produce plasma cells within days.

• The antigen was given in a larger dose the second time: The graph gives no reason to think the dose changed, and the same pattern appears with identical vaccine doses. Memory, not dose, explains it.
• Correct: Memory B cells left by the first exposure responded: Correct. Memory B cells make the secondary response faster and larger.
• Antibodies from the first response were still binding the antigen: Leftover antibody would, if anything, clear some antigen and blunt the response. The rise comes from new plasma cells.
• A different antigen was given the second time: A different antigen would bring a new primary response, with the same lag and low peak as the first. Memory is specific.

When IgG binds a microbe with its arm tips, its Fc region points outward. Phagocytes carry Fc receptor proteins that bind that stem, so coated microbes are engulfed far more readily. This is opsonization.

• The light chains: Light chains form part of the arms, which are busy gripping the bacterium. They do not bind Fc receptor proteins.
• The antigen-binding sites: The antigen-binding sites are attached to the bacterium, so they are not free to bind the macrophage.
• The variable region of the heavy chains: The variable regions sit at the arm tips, on the bacterium. Receptor proteins on phagocytes bind the constant stem.
• Correct: The Fc region: Correct. Fc receptor proteins bind the Fc region.

A second exposure to the same antigen recalls memory B cells. The response starts sooner, goes higher, is mainly IgG, fits better and leaves more memory cells. That is why boosters work.

• Days until antibody rises in the blood: down. Memory B cells need less stimulation and are more numerous than the naïve cells were, so plasma cells appear within 1–3 days instead of 5–10.
• Peak antibody level: up. Many memory B cells each form a clone of plasma cells, so far more antibody is made, often 10 to 100 times more.
• Share of the antibody that is IgG: up. Memory B cells have already switched class, so the response is mostly IgG, with little of the IgM that starts a primary response.
• How tightly the antibody binds the toxin: up. Memory cells come from B cells selected for better-fitting antibody during the first responses (affinity maturation).
• Number of memory B cells afterward: up. Each secondary response makes new memory cells as well as plasma cells, so the pool grows.

Passive immunity is protection by antibody someone else made. Antivenom, maternal IgG, IgA in breast milk and injected anti-D are all borrowed antibody. A vaccine and recovery from infection make your own immune system respond: active immunity.

• Correct: Antivenom given after a snakebite: Correct. Antivenom is antibody made in another animal: artificial passive immunity.
• A measles vaccine: A vaccine makes your own B and T cells respond and leave memory: artificial active immunity.
• Correct: IgG crossing from a mother into her fetus: Correct. Maternal IgG is natural passive immunity.
• Recovering from chickenpox: Recovering from an infection is natural active immunity; your own cells made the antibodies and memory cells.
• Correct: IgA a baby swallows in breast milk: Correct. IgA in milk is natural passive immunity for the baby's gut.
• Correct: Anti-D injected into an Rh-negative mother: Correct. Injected anti-D is borrowed antibody: artificial passive immunity.

Capsule sugars are T cell-independent antigens: they give weak, mostly IgM responses with little memory, and poor ones in young children. A B cell that binds the sugar takes in the whole conjugate and displays peptides from the attached protein on MHC class II. Helper T cells that recognize those peptides then help it expand, switch class and form memory. This is a conjugate vaccine.

• The protein makes the sugar a larger target for IgM: Size alone does not help. The gain comes from helper T cells, which can then drive class switching and memory.
• Protein antigens do not need B cells at all: Protein antigens do need B cells to make antibody. What they add is peptides for helper T cells.
• The protein stops the sugar from being broken down before B cells can reach it: The sugar's lifetime is not the problem. Without T cell help, B cells make a weak response however long the antigen lasts.
• Correct: Helper T cells that recognize the protein can help B cells that recognize the sugar: Correct. Linking the sugar to a protein turns the response into a T cell-dependent one.

This is opsonization. Bound IgG leaves its Fc regions facing out, and phagocytes grab them with Fc receptor proteins, which strongly speeds engulfment.

• The IgG molecules punch holes in the bacterium: Antibodies do not make holes. The membrane attack complex of complement does.
• Correct: Fc receptor proteins on a phagocyte bind the IgG stems: Correct. Fc receptor proteins link the phagocyte to the coated bacterium.
• The bacterium displays its peptides on MHC class I: Bacteria do not have MHC proteins; MHC is found on your own cells.
• IgG enters the bacterium and blocks the enzymes it needs to divide: Antibodies act from outside, on surface antigens. They do not enter bacteria.

Antivenom is passive immunity: antibodies made in another animal. They work at once but are broken down within days to weeks, and because his own B cells did not make them, no memory cells formed.

• Yes, because the antivenom left memory B cells: Memory B cells come only from your own immune response. Borrowed antibodies do not create them.
• Yes, because antivenom antibodies last for decades: Borrowed antibodies last days to a few months, not decades.
• Correct: No, because antivenom is broken down and leaves no memory: Correct. Passive immunity is immediate but temporary.
• No, because the first dose made him permanently immune to the snake's venom: The answer contradicts itself: if he were immune, he would be protected. Antivenom does not make you immune.