Endocrine Systems

T cells also participate in the adaptive immune response. Both B cells and T cells are lymphocytes, differentiating them from other white blood cells in the body. T cells don’t directly recognize a pathogen. Instead they recognize specific antigens that are attached to MHCs (major histocompatibility complexes) that are basically self-identifying complexes. These MHCs are necessary for T cells to recognize the antigen.

When a cell is infected, antigens from the pathogen will be found on the outside of the cell, linked to MHCs that identify the cell as belonging to the self but being infected as well. These antigen-presenting cells will trigger a T cell and a B cell response that is specific to the pathogen. While the B cell will be triggered to make antibodies, the activated T cell will directly kill anything that contains the antigen they’ve been activated to kill. This is called cell-mediated immunity. T cells only kill self-cells that have been identified as being infected or possibly cancerous.

There are cytotoxic T cells that do the killing of infected cells and T helper cells that aid in the process but do not do any infected cell killing. T helper cells will secrete cytokines when in contact with an infected cell. This stimulates NK cells and macrophages to kill the pathogen.

So, how do vaccines work? They are given to humans and animals in order to prevent an infection. These represent killed bacterial cell wall parts, killed viruses, or live but harmless viruses that can generate an immune response against the infection they represent. These vaccines basically contain antigens that allow the adaptive immune system to kick in and to develop a memory for having an infection that the individual never actually had in the first place. This memory prevents the infectious organism from taking hold in the body by tricking the immune system into thinking the person has already had the infection. These will confer a long-term response against infection.

ENDOCRINE SYSTEMS

Some aspect of an endocrine system exists in animals of all types. It represents several glands that secrete hormones. Hormones can be simple molecules, modified amino acids, and peptides that act over long distances to affect the body in some way. Hormones are secreted by a variety of endocrine glands that have no ducts but secrete

the hormone directly into the immune system. Figure 53 illustrates the aspects of the endocrine system that are seen in humans.

In mammals and humans, there is the hypothalamus and pituitary gland, which together secrete most of the hormones that affect the other endocrine glands in the body. There are hormones secreted by the pituitary that affect growth, alter metabolism, generate the sexual and reproductive responses in males and females, regulate blood sugar, and regulate salt and water balance in the body. Some of this is a direct response (as is the case with growth hormone), while most act on other glands (like the adrenal gland and the thyroid gland) that do the actual end-organ action in the body.

The main endocrine glands in the body are the pituitary gland, the pineal gland, the thyroid gland, the parathyroid glands, the adrenal glands, the pancreas, the ovaries, and the testes. The pituitary gland is a pea-sized gland near the base of the brain that secretes these hormones:

• Growth hormone—this regulates the growth of the body.

• Follicle stimulating hormone—this regulates the development of the gametes (sex cells) in males and females.

• Prolactin—this controls milk production in the body.

• Thyroid stimulating hormone—this regulates the output of the thyroid gland.

• Adrenocorticotropic hormone (ACTH)—this regulates cortisol secretion in the adrenal cortex, which is part of the adrenal gland.

• Melanocyte stimulating hormone—this helps to produce melanin, which darkens the skin.

• Luteinizing hormone—this regulates sex hormone production in males and females.

• Antidiuretic hormone—this increases blood pressure and regulates water loss.

• Oxytocin—this contributes to the let-down reflex when milk is secreted by the females that are lactating or breastfeeding.