Recent research has led to Dr Paul Batman publishing a paper in 2022, Physical Activity and the Immune System.  In a nutshell...
 
Significant positive changes in immune system functioning can occur as a result of continual habitual physical activity:
MICT 3-5 times a week for an average of 30-60 minutes per session (for at least 12 weeks for measurable difference).
Moderate-intensity continuous training, MICT, rather than HIIT (high intensity interval training which has been all the rage for a few years), could be more effective in creating positive changes to T-cell functioning and improving immunity. (HIIT can decrease helper T-cell concentration and reduce total T-cell numbers and helper T-cells by 38%!) Whereas MICT increased total T-cell number and helper T-cells by 20%! 

Neutrophils are very potent weapons against infection - but too many can cause damage to healthy tissue. (If you want to improve immunity, don't over-exert yourself with HIIT or equivalent exercise!)  
Improved immunity is associated with a reduction in the accuracy of neutrophil migration and the decreased clearance of bacteria. Neutrophils are key leukocytes that arrive at infected sites and can increase in number from resistance training lasting more than 30 minutes, as well as continuous aerobic training and high-intensity exercise; but only remain elevated for 30-60 minutes post-exercise. This means that we need to exercise DAILY. (Examples could be faster walking, just picking up your pace until your puffing a bit but not so out of breath that you can't string a sentence together.)  The migration of neutrophil is critical in controlling infection, inflammation and wound repair and, if neutrophil does not migrate sufficiently, can cause damage to surrounding tissues by elevated protein breakdown leading to further immobility.  
Inactivity and sedentary behaviour can play a significant role in a further loss of speed and accuracy. Neutrophil performance is highly regulated and dependent on physical activity levels: the more active you are, the greater improvement in neutrophil functioning can be.  
Improvements in aerobic capacity, glucose control and reduced body fat could be the catalyst for improvements in neutrophil functioning.

Physical activity is a powerful mediator for improving immunity during the ageing process as well as offsetting immunosenescence.
With an increase in T-cell mobilisation immediately during and after an exercise bout that tends to be higher in more physically active older adults, indicates the gains are much greater with ongoing participation in all movement.

As they say on TV's "Strictly Come Dancing": "Keeeep dancing!"

If you find it difficult to achieve MICT 3-5 times a week, contact me and let's sort out a healthy programme for you.