Immunity: Cytotoxic Immunity

Cytotoxic Immunity

Adaptive immunity uses antibodies or immunoglobulins(Ig) Antibodies: Proteins produced by the immune system to target specific antigens. to neutralize a pathogen or invader. Cytotoxic immunity focuses on killing the actual infected cell. Unlike humoral immunity, which relies on B cellsThe basic structural and functional units of life. to produce immunoglobulins, cytotoxic immunity employs T cells. These T cells recognize and destroy pathogens.

Types of T cells

T cells come in 15 different types, each with a distinct role. Cytotoxic T cells attack and destroy infected cells, but their actions need to be regulated. Regulatory T cells stop the cytotoxic activity when necessary. Additionally, memoryThe ability to store and recall information. T cells are created to recognize the antigen for future encounters, enabling a faster immune response.

All T cells have T-cell receptorsProteins located on the surface or inside cells that bind specific molecules (e.g., neurotransmitter (TCRs), which identify them as T cells. Naive T cells, which haven’t yet encountered their target, mature into either CD4 or CD8 cells. All T cells mature in the thymus. They undergo positive and negative selection. This ensures that they can recognize foreign antigensMolecules on the surface of cells that trigger an immune response. for destruction. It also ensures that they don’t recognize self antigen and leave them alone. 

In the thymus, T cells that can recognize the MHC Class I antigen are designated. These T cells will become cytotoxic or CD8 T cells.  T cells that can recognize MCH Class II antigens become helper or CD4 T cells. 

Antigen Presentation

The first step in mobilizing the cellular army, if infection is present, is to present the antigen. This is done to naïve T cells. These T cells are just kind of hanging out in the lymph nodes.  An APC, here a dendritic cell, drinks a foreign antigen via pinocytosis and moves to a lymph node.  In the lymph node, the APC displays the epitope of the pathogen on its MHC Class II proteinsLarge molecules made of amino acids with various functions in the body..  This is the APC saying, “Hey, I found this.” 

Clonal Selection

Before the T cell can react, it has to go through costimulation.  This is the process of the T cell binding to a second protein in the cell membrane of the APC.  This costimulation protein ensures that we are not going to induce an immune response with no pathogen present.  Basically, costimulation is just the T cell making 100% sure that it should clone itself.

There are essentially three signals that have to happen for a T cell to become activated. Signal 1 is the binding of the T-cell receptorA structure that detects stimuli. (TCR) to the MHC Class II protein on the APC.  Signal 2 is the binding of the T-cell co-receptor to a similar receptor on the APC surface.  The binding of cytokines to their receptors on the T cell represent signal 3..  Different combination of these cytokines influence T cell differentiation into distinct effector T cell subtypes that produce signature cytokines.

 Once the T cell makes sure that it should go ahead and clone itself, it does, making a huge army.  Among the cells in the army are cytotoxic T cell that will just go forth and destroy.  Regulatory and memory T cells are made.

Cytotoxic CD 8 Cells

The cytotoxic T cell binds to the foreign cell. It pokes holes in the cell. Then it injects enzymesProteins that speed up chemical reactions in the body. and toxins into it. Finally, it breaks up the cell membrane. Cytotoxic T cells use proteins like perforin to combat invaders. Perforin creates pores in the cell membrane of infected or pathogenic cells, leading to their destruction. In viral infections, perforin causes the infected host cell to burst. Just as a dying, virally infect cells, cytotoxic T cell release interferons.  TNF or tumor necrosis factor, which activates macrophages, is also released, especially when cancer cells are encountered

Additionally, cytokines can induce apoptosisProgrammed cell death, an essential process for growth and development., or programmed cell death, in nearby infected cells. Lymphotoxins, another tool used by T cells, disrupt ATPThe energy currency of cells used for muscle contraction. production, further incapacitating infected cells. In the end, the immune response relies heavily on CD8 T cells to do the actual killing.

Helper
CD 4 Cells

CD4 cells, or helper T cells, facilitate recognition of infections. CD8 cells kill infected cells or pathogens. Meanwhile, CD4 T cells, known as helper cells, coordinate communication between various immune cells.

Helper T cells will go and speak to APCs and say, “Hey, we are looking for this.  If you find it, let me know.”  As the helper T cell communicates with an APC, it releases cytokines. These cytokines draw close NK cells and leukocytes. They start inflammation and invoke other innate defenses, like fever, which could be helpful.  Helper T cells tell B cells to produce antibodies and stimulate CD8 T cells to destroy infected cells. Cytokines act as distress signals, calling for help to eliminate the invader.

When a helper T cell encounters an APC with an antigen, it releases interleukins that perform three functions. They attract neutrophils and NK cells. They also attract and sequester macrophages. Additionally, they stimulate B and T cell mitosis.

Memory T Cells

Every time we use adaptive immunity, we generate memory cells. Both CD8 T cells and CD4 T cells contribute to the immune response. They create memory cells and increase their numbers.  These memory cells are longer lived and can activate more quickly.  This ensures that the next time we encounter the same pathogen, the immune system responds quickly and efficiently. It’s as if the immune system says, “I know who you are.” It knows what cells to activate to eliminate the pathogen.  This more rapid response is called a T cell recall response.