Adaptive Immune System

What is cell-mediated immunity?

Immunity dealing with invaders living in a body cell; relies on T lymphocytes

What is humoral immunity?

Immunity that eliminates pathogens in blood or other fluids; relies on B lymphocytes that produce antibodies

What are T-cell receptors?

Receptors that only bind an antigen “presented” by one of the body’s cells

What are B-cell receptors?

Membrane anchored antibodies that bind free antigens. They look like a Y, where the two arms are identical to each other and thus have two antigen binding sites

How are naive B cells activated?

• naive B cell encounters antigen and takes it in with its B-cell receptor

• B cell degrades antigen and presents it on MHC class II molecules

• CD4 T cell binds to the antigen-MHC II complex on the B cell

• CD4 T cell then delivers cytokines to the B cell to activate it 

• Activated B cell proliferates and turns into antibody-secreting plasma cells and memory cells

What happens after a B cell is activated into a plasma cell?

Plasma cells make antibodies, which are secreted versions of the B cell receptor, and they have the same binding specificity as the B cell receptor of the original B cell

Describe an antibody

• Y shape

• two identical arms: Fab region containing the variable region

• stem: Fc region containing the constant region

Five general constant regions on antibodies

• IgM- produced during primary immune response

• IgG- passed from mother to infant, most abundant in the blood, produced during secondary immune response 

• IgA- important in mucosal immunity

• IgD- helps with antibody maturation

• IgE- heavily involved in allergic response

What are the outcomes of antibody-antigen binding?

• neutralization- prevents pathogens from binding

• opsonization

• complement system activation

• immobilization of the pathogen

• cross linking- the two arms of the antibody bind two separate antigens

How do T cells get activated?

Naive T cell with a T cell receptor recognizes an antigen on an MHC complex. Then, it proliferates and differentiates accordingly. 

What cells have MHC II? 

B cells, macrophages, and dendritic cells

What cells have MHC I?

All nucleated body cells

What happens when a naive T cell encounters an antigen presented on MHC II?

• pathogen gets engulfed by a macrophage or dendritic cell (after being recognized by a PRR), or a B cell (after binding to the B cell receptor)

• antigen is presented on the MHC II of B cell, macrophage, or dendritic cell

• T cell receptor detects this and differentiates into CD4 T cell

• CD4 T cell releases cytokines so that B cells can make antibodies and other immune cells are called 

What happens when a naive T cell encounters an antigen presented on MHC I?

• body cell gets infected (by a virus or bacteria) and presents antigen on MHC I

• naive T cell gets activated and differentiates into CD8 T cell

• CD8 T cells release cytotoxic molecules to kill infected cells or trigger apoptosis

What are T-dependent antigens?

When recognized by B cells, the B cells cannot be activated without the help of a CD4 T cell

What are T-independent antigens?

When recognized by B cells, the B cells can be activated without the help of a CD4 T cell. They include things like LPS and specific carbohydrates

What are epitopes?

Distinct region of an antigen that the adaptive immune system recognizes

What do natural killer cells do?

• they induce apoptosis in antibody-bound body cells 

  • they have Fc receptors that bind to the Fc portion of an antibody, so they can kill cells that are infected and covered in antibodies

• they recognize host cells that lack MHC I