Topic 2: Cells: Cell mediated immunity

Lymphocytes carry out a targeted (specific) attack on the pathogen

Slower than non-specific defence, but they produce a unique response for each type of pathogen. They also provide long-term immunity against specific pathogens

T lymphocytes (T cells)

Develop from stem cells in the bone marrow and mature in the thymus. Involved in the cellular response where they respond to antigens presented on body cells.

B lymphocytes (B cells)

Develop from stem cells in bone marrow, lymphatic tissue, spleen and lymph nodes and mature in bone marrow. They are involved in the humoral response where they produce antibodies in body fluids (or humours)

When T cells mature

Some become helper T cells and others become cytotoxic T cells

T cells respond to

Antigens presented on the surface of our own cells (APCs)

B cells respond to

Free antigens found in our blood plasma and tissue grid

APC - infected body cells

Will present the viral antigens on their surface on MHC molecule

APC -macrophage

Which has engulfed and destroyed a pathogen will present the antigens on their surface

APC - Cells of a transplanted organ

Will have different shaped antigens on their surface compared to your self-cell antigens

APC - Cancer cells

Will have abnormal shaped self-cell antigens attached to MHC

T helper cells (B cells and phagocytosis stimulated using interleukins)

Once T helper cell has been activated/ become competent, because the complementary antigen on the APC has been presented and binds to its receptors, it then divides by mitosis to produce large quantities of itself. Stimulates phagocytosis. Activate cytotoxic T cells, which go on to kill infected body cells. Develop into memory cells. Stimulate B cells to divide

T regulatory cells

Helps to control immune response once pathogens have been destroyed. This is why you do not keep producing loads of cytotoxic and helper T cells once the infection has cleared. If not working well leads to autoimmune conditions ( immune system attacking body's own cell)

Naïve T cell

Yet to come in contact with complementary antigen. Once in contact with complementary antigen develop into T helper and T killer cells

Cytotoxic T cell

These cells kill abnormal and foreign cells by producing a protein known as perforin. This protein makes holes in the cell-surface membrane, causing it to become freely permeable and causing cell death. Granzymes enter the cell and break down proteins, lysing the cell. This is most common in viral infections because viruses infect body cells. Body cells are sacrificed to prevent viral replication e.g. When you get a sore throat from a cold.

Memory T cells

Provide long-term immunity against specific pathogens. They provide a rapid response if the body is re-infected by the same pathogen (secondary immune response). Long-lived and can rapidly differentiate into cytotoxic T cells if infected with same pathogen again

1st step in cell mediated response

Once a pathogen has been engulfed and destroyed by a phagocyte , the antigens are positioned on the cell surface - APC

2nd step in cell mediated response

T-helper cells have receptors on their surface which can attach to the antigen on APCs

3rd step in cell mediated immunity

Once attached this activates the helper T cells to divide by mitosis to replicate and make large numbers of clones

4th step in cell mediated response

Cloned helper T-cells differentiate into different cells:

• some remain as T helper cells and activate B lymphocytes (by producing interleukins)

• Some stimulate macrophages to perform phagocytosis (by producing interleukins)

• Some become cytotoxic T cells

• Develop into memory cells

• Activate cytotoxic T cells - this allows them to kill infected cells

Why is this called all mediated response

Because T cells only respond to antigens which are presented on own body cells (APC), and not antigens detached from cells and within body fluids, such as the blood. Immunity involving body cells.