Antibodies and B Cells

Helper T cells or Th cells have the CD4 molecule in them. They secrete cytokines (chemical

messengers). There are two types of Th cells. The Th1 cell makes cytokines that regulate the

immune response by acting on other T cells and macrophages. The Th2 cell also secretes

cytokines but these act on B cells so they can turn into plasma cells that make antibodies. T

helper cells are necessary for most situations where an antigen goes on to make antibodies.

Cytotoxic T cells or Tc cells do the actual killing of infected cells in the same way as NK cells do.

They either express the Fas ligand (that binds to Fas receptors on infected cells) or make

perforins and granzymes to cause apoptosis from within the cell.

Regulatory T cells or Treg cells are also referred to as suppressor T cells. These have several

molecules on their cell surface. The first is the CD4 molecule (making them similar to helper

cells). The others are called CD25 and FOXP3. It is believed that they suppress clonal expansion

so that the immune process does not go unchecked. It is a cell type that isn’t completely

understood.

ANTIBODIES AND B CELLS

Antibodies are also referred to as immunoglobulins. They are made by B cells in response to a

specific antigen. They are actually receptors on B cells that get secreted into the blood stream

in most cases. Some are not secreted but are called surface immunoglobulins on the B cell.

There are five kinds of antibodies, called IgM, IgD, IgG, IgA, and IgE. Each antibody type acts

slightly differently in the immune system that are effective in killing pathogens.

Remember that B cells differentiate and mature mainly in the bone marrow. They undergo

central tolerance, which is the destruction of any B cells that recognize self-antigens. Clonal

deletion happens to immature B cells that bind too strongly to self-antigens (preventing the

problem of self-antibodies). It causes these cells to die off. In clonal anergy, the same thing

happens to the B cell; however, the B cell isn’t deleted but is unable to function.

Remember, too, that mature B cells do not have the ability to make antibodies until they get

the go-ahead from Th2 (helper T cells). If the B cell does not get a signal from the Th2 cell, it

gets a signal to die and undergoes apoptosis. This is called peripheral tolerance. Once the B

cell gets a signal, it becomes a plasma cell, divides, and makes antibodies for a specific period of

time before dying off. Only memory B cells (made during the differentiation process) are long-

lived and maintain a memory of the infection.

Antibodies are made from heavy and light chains. Figure 94 shows what the different types of

antibodies look like:

The heavy chain is what defines the type of antibody being talked about. The light chain forms

part of the antigen-binding site. There are two identical heavy chains and two identical light

chains with a Y-shaped antibody structure. The Fc region is the part of the two heavy chains

that are connected to one another. It does not bind to the antigen. The light chain and part of

the heavy chain are bound together in the upper part of the “Y” to form the antigen binding site

(which binds to the antigen). There are two antigen binding sites per antibody molecule.

The antibody can be bound to a B cell, making it a B cell receptor. It can also be secreted into

the bloodstream as a free antibody. Mature B cells that leave the bone marrow can express

both an IgM antibody and an IgD antibody. Of these, only the IgD stays on as a B cell receptor.

These are the different types of antibodies made by B cells:

• IgM—this is a large antibody made from five different antibody segments, making a

total of 10 identical antigen-binding sites. It is the first antibody made in response to an

infection. It starts the process of fighting off the infection by activating the complement

cascade. The B cell, however, can undergo “class switching,” making other types of

antibodies to the same antigen.

• IgG—this is a late response antibody that occurs after antibody switching. It is a single

antibody that can eliminate pathogens in the bloodstream. It can cross the placenta,

providing temporary “passive immunity” to the fetus and baby.

• IgA—this can exist as a four-chain or an eight-chain antibody. It is found in mucous

membranes, acing on body surfaces. It is secreted in breast milk to prevent disease in

infants and providing passive immunity (which is always temporary).

• IgE—this antibody is linked to allergies and to anaphylactic reactions. It causes

degranulation of mast cells, which results in the allergic response, which can be severe.

• IgD—this is a poorly understood antibody expressed in immature B cells. It is

accompanied by IgM cells and is both bound to the B cell as a B cell receptor and

secreted by the B cell in small quantities. They seem to function in allowing the

activation of the B cell to make other types of antibodies.

It takes several days for an antibody to be made against an infection, during which time, the

person is usually sick from the infection. Then IgM antibodies are made, allowing for the ability

to fight off the initial infection. Later, there is class switching so that the IgG antibody is made.

The memory cells are present indefinitely, however, so the next time the pathogen is

encountered, the B cells can proliferate so that the antibody can be directed and the person

does not get sick, or as sick, with future exposure.

Passive immunity involves giving a fetus the antibodies necessary to fight the infection OR to

give a person some immunoglobulins to a specific infection so they don’t get sick from a known