Adaptive Immune System

How do B + T cells arise? (4)

• Derived from haematopoietic stem cells, they form in the bone marrow

• They divide into common lymphoid progenitors (CLPs)

• Cells left in the bone marrow are B cells and those that migrate to the thymus are T cells

• B cells mature in the bone marrow and then migrate

How do B cells mature?

Before maturing B cells have gene segments of immunoglobulin (IgM) rearranged so a unique cell surface IgM (BCR receptors) is formed.

What is Ig and what classes are there?

Igs are antibodies and there are 5 classes;

• IgM - A pentamer abundant in mammals, provides specific immunity

• IgG - also abundant, same function

• IgE - defends against multicellular parasites and is in allergic responses

• IgA - A dimer which is secreted in linings to protect locally and is in milk

• IgD - unclear

Function of antibody

Recognise antigens/ parts of antigens and binds to them (epitope)

What is an antigen and what kinds are there?

An antigen is a substance capable of eliciting an immune response (but won’t always). It includes;

Exogenous

• Foreign antigens (heteroantigens)

Endogenous

• Self antigen (autoantigens)

• Cancer derived antigens (neoantigens)

Immunogens

• Complete antigens capable of eliciting a specific immune response (macromolecules)

Haptens

• Incomplete antigens which do not generate an immune response by themselves (low molecular weight)

• Requires a carrier molecule

How do B cells fit into adaptive immunity?

B cells are activated by antigens and T helper cells. Once activated B cells proliferate into two cell types:

• Plasma Cells

  • Cells that attack the specific antigen their BCR targets by producing antibodies

• Memory cells

  • Cells that remain after the immune response to respond to later infections with the same specific antigen

In the case of T-dependent activation, the B cells that proliferate express the same receptor.

B cells also present antigens.

How is T cell diversity generated?

T cells arrange their receptor gene segments. T cells which recognise self MHC survive while T cells that interact too strongly with MHC are removed via apoptosis (selection). The T cells differentiate into two cell types: CD4 Helper cells and CD8 Killer cells.

The mature T cells move to the periphery lymphoid organs and when activated act upon antigens.

Progenitor T-cells produce multiple lymphocytes which all recognise different antigens (through recombinant genes). Lymphocytes that proliferate all express the same receptor (T Cell Receptor, TCR).

How are antigens presented to T cells?

Naive T cells circulate around the body for their specific antigen, once an interaction forms TCR sends signals to activate the T cells.

Broken down antigen by phagocytosis undergo acidification to break down peptide segments and presented to MHC molecules. MHC class I differentiates T cells into CD+ 8 Killer cells and the antigen is presented inside the cell. MHC class II differentiates T cells into CD+ 4 Helper cells, MHC class II is selectively expressed on APCs.

Explain T cell functions

The effector functions of T cells are:

• Cytotoxic CD+8 Killer cells

  • Attack infected cells and induce apoptosis

• CD+4 Helper cells

  • Th1 - secretes IFNy and activates macrophage function

  • Th2 - secreted IL-4 and helps with antibody production.

Some T cells remain and act as immunological memory for secondary responses. These memory cells require consistent stimulation of IL-5 and IL-7 to proliferate and further signals.